Cyparagon and Schrecken Licht have CO2 lasers. You basicly need the tube, a water cooling system for the tube , and of course a psu(for the tube and the cooling system).And some eye protection and a lot of carefulness :
you don't see them in the DIY forums here because they are more expensive than most care to spend even for a homebuilt. Plus you cant put them in your pocket and take them with you hence Laser Pointer Forums
They are not that expensive especially when considering $/mW. Mine was about 2 cents/mW. The tube is damn near impossible to make (let alone align and fill). So that's pretty much out of the question. The other aspects, however, can be completed yourself.