Can get 3.5oz. tanks from a few paintball on-line shops. Are not cheaper than the larger tanks.
There are also the same tanks with a manual on/off ( a knob built in the valve that you manually turn) but they run about $40-$45.
Do NOT get the 4oz. tanks. These are as fat OR FATTER as 9oz. tanks and will have the same fit problem.
Some time back, had Mike make me a .177 Qb78 shooting that fast. It really did, mike is good about the statements he makes.
After a bit, decided to tune it back down a bit. Takes good bit of co2 per shot to get up to 800fps, even in .177. I'd say you'd get something close to 40-45shots per ounce of co2...call that 155-160 shots for a 3.5oz. tank and 400 for a 9oz. (assuming no leaks and summer time shooting).