do you have a bottom line assembly on the gun already? if it has the but to screw the bottle into and some hosing going into the gun, then you're away. just stick a hopper on top and youre good to go
o-rings are cheap and you'll go through quite a few, either go to your local site (99%sure they'll have them-especially if there a "punter" site) and the bottle wont work without one.
if that fails, local plumbers all the way