I just bought a tank after spending a few weeks looking at eBay and several air gun forum classifieds. I bought an Interspiro 1/2012 DOM, 66 ft3 SCBA from airtanksplus.com. It's well used and came with a typical fire fighters Scott or CGA-347 valve, ready to use. I've bought a SCBA Hose Fill adapter elsewhere and soon as that arrives,I'm ready to go find some air....I looked for a bigger tank, but they got real pricey, real quick. If I stay in this, my next tank will probably be a new or near new tank. In the meantime, this tank will do fine.
Be cautious on eBay, you have to know what you're buying cause most sellers don't care if you get stuck with a tank you can't get filled. If the DOM isn't clearly shown in the ad, go elsewhere. eBay tanks get expensive quickly when they have only a few couple of good years of life left. Mine was far cheaper for many many more years of serviceable life. If you have your own compressor (I don't...), eBay becomes a much better deal, buy as big of a tank as you can find that's just expired with a recent hydro and enjoy!