try ebay! www.ebay.com,

you can get pretty much anything cheap on ebay, the postage will probably work out more expensive than the actual computers.

another area to look at is thin clients, there often sold on ebay for a few dollars (that sounds so wrong, i should be using pounds), and there network enabled, not as widely documented though

a few other things you could try is emailing large local companys and say your woundering whether they have any older hardware that would otherwise go to waste as your a poor student and want to build a cluster to develop your knowledge. Try bootsales (garage sales/ yard sales?) youll find allot of people have used computer "junk" they want to get rid of. Put an advert in your local paper saying you require old computers for your...education.. and see if you have any takers, say youll take them off there hands for a small sum of money. be careful how you word it though.

Try your college or universtiy as well, my mate was going to get given 20 pent II's when his school upgraded cos he does some system admin for them but they decided to hamper his plans of building a cluster by sending them to africa to allow the kids there to surf the net and use comptuers (very good alternative i reackon).

i2c