Where to buy computers cheap  

If I were you I would use google and see what you can come up with
Or try classified adverts in the area you say in. But there are bound to be endless listings of sites selling what you want. So, I would do it like that

Well, you can try on eBay. Not all computers that are sold there are really old, some people just get tired of their new computers soon and put them for selling on ebay. I think is the place where you can find a cheapest one. There or in a secondhand shop in your city if you have one.