I went with a 237 BAR (3440psi) 100 cu ft steel SCUBA tank. I shoot my Marauder down to 2000 psi before filling. I get 20+ fills to 3000 psi, good for three magazines, but I usually keep using the tank till it's down to about 2500psi before I get it refilled. That gives me another 60+ "two magazine" fills. It's not something you want to haul out to the field with you though!
The carbon fiber SCBA tanks are nice, and light, but have a 15 year service life before they're scrap. That kinda precludes finding an inexpensive used one as most that I found were at, or close to, their expiration date.
