This thread is over a year old, but I just noticed it again.
From what I’ve heard, the best place to look may be eBay. That will necessitate some savvy on the part of your IT person. You’ll need to know the kind of specifications your use calls for, and be able to do some basic troubleshooting. But since many manufacturers use parts of similar quality, it isn’t too important that all your computers be the same model.
I’m told that educational discounts on new computers still mean paying more than going with used.