In closets, attics and storage rooms all over America, perfectly good computers are gathering dust because their owners upgraded to new systems but couldn't find anything to do with the old PC.

And in offices, churches and homes all over America, people are doing without the computers they need because they can't afford to buy a new PC and can't find a good used one.

Hooking this supply up to the demand is the job of the used-computer market, a rather haphazard industry situated here and there around the country. With Americans buying millions of computers and peripherals each year, it is inevitable that there are large numbers of used machines. How can you find them?

Nearly every newspaper has a special section for "Computers" in its classified ad section, and you might find the machine you need there. But this is a chancy proposition.

An obvious source of used computers is big companies, which often replace hundreds of old PCs at a time. It is common practice at many firms to sell these machines at bargain prices to employees. It might profit you to find out if your company does this. And it might profit your company if you convince the brass to start such a program.

The closest thing to a national organization in the used-computer market is the National Association of Computer Dealers (1-800-223-5264). NACD acts as a coordinating arm for a loosely knit amalgam of dealers around the country who trade in used computers, printers and disk drives. The group turns out various publications that help sellers to unload their used computers and buyers to find them.

Each March, NACD publishes a "Computer Blue Book" (available in some book stores at $15.95), listing suggested prices for about 18,000 second-hand computer products in four regions of the country.

A recent edition lists, for example, a price of $2,348 for a used Dell Computer 16-megaHertz 80386 PC with a monochrome monitor, one floppy and one 70-megabyte hard drive. Not a bad deal, but you could get a brand-new '386-16 for about the same price from several mail-order houses. The Blue Book says a used Epson LQ-500 24-pin printer runs $214, some 35 percent lower than mail-order prices for new 24-pin printers.

A bigger but much more expensive blue book of used PC prices is available from Orion Research (303-247-8855) for $125.

An outfit called Link House Publications (1-800-322-5131) puts out a magazine called Computer Hotline. It's $99 per year for the weekly edition, or $39 per year to get one issue a month. Link House will generally send one issue free if you want to see what Hotline looks like.

Computer Hotline, about as thick each week as Time or Newsweek, contains nothing but mail-order computer ads. Unlike the big computer mags, Hotline has countless ads from firms selling used computers.

Prices for used stuff in Hotline tend to run below what you would pay for new goods through the mail-order. A recent issue had an ad for the Compaq Portable III with a 20-megabyte hard disk for $1,450. That's a good deal as long as you're willing to have a machine that is behind the state-of-the-art. Another ad offered a refurbished 20-megabyte Seagate hard drive for $129, the best price I've ever seen for that device.

Retail stores specializing in used computers are starting to appear in major cities. Their prices tend to be pretty low.

The nice thing about buying used computers from a nearby store is that you have a place to go back to if it blows up the second day you own it. Nobody is going to give you a full new-equipment warranty on used hardware. But most stores will provide 30 to 90 days' coverage, long enough to uncover any serious glitches.

Another way to find used PCs is to fire up your modem and call a local (or national) bulletin board system. Many of them have special sections for folks offering to sell their old machines.

On national call-in systems like CompuServe, MCI Mail and Delphi, you can contact the Boston Computer Exchange (on CompuServe, "Go Bocoex"; voice, 1-800-262-6399). This purports to be a national listing of used computer gear. The exchange charges $25 for each listing, and then charges the seller a further fee if the item sells. Thus, the service itself is free to buyers, who pay only the asking price.

Finally, here's a grand option for folks who have old PC gear sitting in a closet. The National Cristina Foundation (1-800-CRISTINA) acts as a clearinghouse for charitable outfits that need computers. Through them, you can donate your used PC to a worthy cause, get a tax deduction and clean out the storage closet -- all in a single phone call.