When you find one, as was mentioned, research the bulb. Some of the older bulbs may no longer be available, or at very high prices. I remember seeing discussions about converting an old Rollie projector to use a different bulb, because of that reason, the bulb could not be found.
Lots easier to find a 6x6 projector, but manual 6x7 projectors are out there. Be patient is the key, watch eBay or the local internet classifieds (similar to Craig's List). It took me over a year to get my 6x6 projector.