Being patient with eBay to snag a cheap listing once it comes up, searching local online classifieds (like Craigslist) occasionally, visiting recycling centers (depends on the area), and thrift store hunting are some of my secrets for finding stuff like this. You might not see one for sale right away too easily, but being patient and searching thoroughly whenever you have the chance to can help you find something useful.
eric1992 wrote:And did Gateway use proprietary motherboards?
To my knowledge, Gateway computers from BITD used standard MicroATX motherboards without the catches other brands were known for pulling off (so you won't have to deal with proprietary PSUs and connectors). However, most Gateway computers (including the Essential 433 tower that I had) do have the I/O plate integrated onto the case so my suggestion for that hurdle is to either look for a motherboard with the matching connectors or cut out an opening for an I/O shield plate on the back of the case.