Eric Spiegelman has an interesting post on how the legal establishment got the individual mandate so wrong. In it, he writes:
How far can the definition of Congress' enumerated powers be stretched? As Justice Scalia asked during oral arguments: if Congress can force you to buy health insurance, can they also force you to buy broccoli? The question I like to ask is: what if Congress forced you to buy a gun?
But Congress has forced Americans to buy guns. It's in the Militia Acts of 1792. The relevant section is a bit lengthy, so I've bolded the key parts:
Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That each and every free able-bodied white male citizen of the respective states, resident therein, who is or shall be of the age of eighteen years, and under the age of forty-five years (except as is herein after excepted) shall severally and respectively be enrolled in the militia by the captain or commanding officer of the company, within whose bounds such citizen shall reside, and that within twelve months after the passing of this act. And it shall at all times hereafter be the duty of every such captain or commanding officer of a company to enrol every such citizen, as aforesaid, and also those who shall, from time to time, arrive at the age of eighteen years, or being of the age of eighteen years and under the age of forty-five years (except as before excepted) shall come to reside within his bounds; and shall without delay notify such citizen of the said enrolment, by a proper non-commissioned officer of the company, by whom such notice may be proved. That every citizen so enrolled and notified, shall, within six months thereafter, provide himself with a good musket or firelock, a sufficient bayonet and belt, two spare flints, and a knapsack, a pouch with a box therein to contain not less than twenty-four cartridges, suited to the bore of his musket or firelock, each cartridge to contain a proper quantity of powder and ball: or with a good rifle, knapsack, shot-pouch and powder-horn, twenty balls suited to the bore of his rifle, and a quarter of a pound of powder; and shall appear, so armed, accoutred and provided, when called out to exercise, or into service, except, that when called out on company days to exercise only, he may appear without a knapsack.
Now, you could argue that this was not done under the Commerce Clause. But as Yale's Akhil Reid Amar says, "the law shows that George Washington, who signed the law, thought that purchase-mandates were not intrinsically improper. If Congress can regulate a 'well-regulated' militia with a mandate, why can’t Congress regulate interstate commerce the same way?"
Incidentally, that's not the only time an early congress mandated that Americans purchase privately sold products:
In 1790, the very first Congress—which incidentally included 20 framers—passed a law that included a mandate: namely, a requirement that ship owners buy medical insurance for their seamen. This law was then signed by another framer: President George Washington. That’s right, the father of our country had no difficulty imposing a health insurance mandate.[...]
Six years later, in 1798, Congress addressed the problem that the employer mandate to buy medical insurance for seamen covered drugs and physician services but not hospital stays. And you know what this Congress, with five framers serving in it, did? It enacted a federal law requiring the seamen to buy hospital insurance for themselves. That’s right, Congress enacted an individual mandate requiring the purchase of health insurance. And this act was signed by another founder, President John Adams.
That's from Einer Elhauge, a professor at Harvard Law, who continues, "not only did most framers support these federal mandates to buy firearms and health insurance, but there is no evidence that any of the few framers who voted against these mandates ever objected on constitutional grounds. Presumably one would have done so if there was some unstated original understanding that such federal mandates were unconstitutional."
Also of note: unlike the mandate to buy muskets, the maritime mandates were exercised under the Commerce Clause.