The fantasy find for historian Lonnie G. Bunch III includes a tattered pair of pantaloons made of that old “Negro cloth” and a coarse linen shirt that all but disintegrated on the back of its enslaved owner. Maybe a thrashed pair of brogans, too, worn around the plantation until the soles fell off.

Slave clothing,” Bunch said, almost wistfully, of the most elusive item on his historical wish list. “Still can’t find any.”

Bunch, the founding director of the National Museum of African American History and Culture, is tantalized by the possibility that somewhere — in an attic or basement — somebody has work clothing once worn by a black slave.

He desperately wants to find, authenticate and restore it, then use the potent symbol of a painful past to humanize the story of slavery. But he and his staff of historical hunter-gatherers haven’t come close.

“Not at all,” Bunch said. “People will call me and say they have something — and it’s just a pair of Lee jeans made in 1925.”

He sighed.

As the Smithsonian Institution prepares to break ground next month, a critical curatorial deadline looms for the first national museum devoted to black American history: 90 percent of its collection should be acquired by the end of this year, Bunch said, to leave enough time to create, revise and finalize the inaugural exhibitions.

“By early in 2013, we’ve really got to shut it down,” he said of the historical treasure hunt. Acquisitions will continue after that, Bunch said, but the majority of the late-arriving artifacts won’t be displayed when the museum opens on Constitution Avenue, in the shadow of the Washington Monument, in late 2015.

So seven years after his appointment, Bunch is borderline obsessed with finding the unfindable for the centerpiece “Slavery and Freedom” exhibition planned for the 350,000 square-foot museum’s lower level.

“That’s the spine of the museum, the most dramatic moment,” he said in an interview at his headquarters, on the seventh floor of a nondescript building at L’Enfant Plaza, about a mile from the museum’s future home.

Building the museum’s collection from scratch, a staff of 13 curators has acquired some significant slavery artifacts, including wrought-iron shackles and a slave cabin from Maryland’s Eastern Shore.

“We even have a whip and some other instruments of slave torture,” Bunch said. “We’re going to alienate some people, but it’s about giving the general public an understanding of what the institution of slavery meant and still means. It’s not just the African American story; it’s the story of America.”

Collecting has long been part of Washington’s DNA. The region is rich with museums (there are dozens in the District, according to the American Association of Museums), each of them a repository for thousands of artifacts and dreams of unearthing more.

Of course, no institution here has amassed a larger collection than the Smithsonian, whose network of 19 museums collectively owns more than 137 million items. Its shelves boast everything from Albert Einstein’s pipe to 100,000-year-old fossilized sloth dung. Not for nothing is the Smithsonian often called “America’s Attic.”

Bunch expects the $500 million African American history museum to open with about 25,000 artifacts, encompassing everything from slavery, segregation and the civil rights movement to popular culture and black communities.

Already, the nascent collection is overflowing with treasures. There’s the original coffin in which Emmett Till was buried after his brutal 1955 lynching, and a 1922 Pullman railroad car partitioned, per Jim Crow laws, into separate sections for whites and “colored” passengers. There’s a pair of Ku Klux Klan robes and an open-cockpit biplane — a PT-13 Stearman — once used to train Tuskegee Airmen, the all-black World War II Army corps.

The museum also has a rare collection of photographs of famed writer James Baldwin ; Louis Armstrong’s Selmer trumpet; a replica of the iconic, 1,200-pound Parliament-Funkadelic stage prop known as the Mothership, and a trove of rare Harriet Tubman artifacts, including a silk and linen shawl given to the famous abolitionist by Queen Victoria.

Notably, the collection also features two items worn by an enslaved girl — a bonnet and dress, which were part of the Black Fashion Museum Collection that was donated to the Smithsonian five years ago.

“But it’s more of a Sunday dress that an enslaved woman would sew for her child,” Bunch said. The items’ provenance was documented by a previous owner and deemed authentic by museum curators.

Bunch’s obsession is with “day-to-day stuff” worn by slaves at work, he said: “The hat you would wear to keep the sun off your head, those old brogans made in Lynn, Mass., and sold south, things like that.

“The hardest thing with something like slavery is humanizing the story. It’s like the Holocaust; how do you think about 6 million people? But you can think of a family or individual if you humanize it. I want people to understand what slavery was really like for the people who lived it. It’s important that I find clothing to do that.”

A few articles of clothing assumed to have been worn by slaves are known to exist in the South. The Charleston Museum in South Carolina, for example, has two aprons and some jockey silks, said Jan Hiester, the museum’s curator of textiles. There’s also a piece of a brogan-style shoe that was discovered behind a kitchen wall, she said. “It could very well be a slave-worn work shoe, but I don’t have any documentation.”

Will the African American history museum’s treasure hunters ever come so close?

“Fat chance,” said Susan Eva O’Donovan, a University of Memphis history professor. Slaves, she said, wore clothing until it was in tatters, then they used the remaining cloth for patchwork quilts or sold it to the rag man. Nobody was likely to have saved it, she said.

“Lonnie Bunch is never going to find it, and if he does, it’s going to be an absolute miracle. And even then, how would he know it was a slave’s? It’s not going to have a name sewn into the neck. I guess he ought to dream. But yeah, right. Good luck. He would be more likely to find a part of a slave ship.”

As it turns out, Bunch wants one of those, too: His curators are searching from Africa to North America — and deep oceanic points in between — for at least a piece of a ship that was used in the transatlantic slave trade. They’re more optimistic about finding a “slaver” than they are about unearthing a slave’s work wardrobe.

According to Seth Rockman, a Brown University professor who is researching a book on the national economy of slavery, the fabric and ready-made clothes sold to slave owners in the 18th and 19th centuries were commonly called “Negro cloth.”

Most slave owners distributed either bolts of fabric or ready-made clothes twice each year, in the spring and fall, Rockman said. A typical provision was one of everything: frocks, pants, shirts and the like.

“It was basically disposable,” he said. “This stuff simply isn’t going to exist anymore.”

Still, Bunch refuses to give up, even if the realist in him acknowledges that he’s probably chasing a historical ghost.

“The odds are you probably can’t find much,” he said. “But then again, I never thought we’d find stuff from Harriet Tubman. Part of my job is to believe and help the staff believe and help the public believe. I believe we’ll find it.

“Just don’t ask me how.”

A temporary exhibit on slavery and Thomas Jefferson, curated by the National Museum of African American History and Culture and the Thomas Jefferson Foundation at Monticello, opens next Friday at the National Museum of American History.