Your best bet is to hit up one of the various resellers. These guys, like Northbay Netoworks in the Bay Area, buy up unused and distressed assets from companies and then resell them on the open market. They will hit up someone like me when we have 100 - 150 "old" servers we want to get rid of, and someone like you when you want to buy a "couple of racks worth" of servers to try something out. Servers that are 4 - 5 years old can be had for $400 - $500 per server caveat they may or may not have memory in them. (which is a problem if they take DDR2 memory which is no hard to find).

Conversely there are places like auctionbdi.com which liquidate old buildings and will sell a pallet worth of old servers for $50 - $500 but you really need to go there first to look at them to get a good estimate of how many are recoverable out of the stack. Often times you'll be able to create maybe a dozen "good" servers out of a stack of 50 "liquidated" servers of identical or nearly identical type.

The post liquidator market is a place like "Weird Stuff Warehouse" (sunnyvale) which buys pallets of stuff and pulls out the resalable stuff into their shops.

If you want exactly 1 machine then a card on the bulletin board of a hacker space saying what your minimum requirements are is a good investment, you can often get one machine for free from someone who is trying to get rid of an old machine (I've "donated" several machines that way)

And of course you can create a virtual machine on an existing desktop and just play around with concepts before you get physical hardware.