C.M.
Ebay. I've gotten 4 - that we use in our business - on there. We stick to IBM T40 or higher and they can run about $250 - $400. They're workhorses and are very stable.
***Also - they are what most large corporation use (I had one when I worked for a large bank). Something has to be said for that. On the new side these things are $1500+ machines.
good luck!
C.