Check craigslist for Co2 tanks. I was at the grow place. I needed one. I was impatient and the local guy on craigslist had one for $75, I was going away and could not wait - it cost me $128 at local supply store. Sucks.
I looked at some of their tanks and it looks like they had some used ones they bought from bars or something.
Those bags do not see to work. I got a valve on eBAY for about $38.