3Skull and Rap4 both make high-capacity 450-round hoppers for the A5/X7, but they're kind of blimps and make for a big, heavy target on top of your gun.
Personally I preferred the standard X-7 low-profile hopper. I think they come standard on the current generation of A5s as well.
It's a bit more expensive, but Pinokkio makes a 450-round hopper with an extended nose. It's also available in transparent smoke, so you can see how many balls you still have in there.