Any of the following:-
A) A paintball site. If you aren't playing there, though, they may charge you.
B) A diving shop with the appropriate connector
C) A diver who has their own cylinders. You will need that connector, though...
D) Buy a diving cylinder and...that connector... It's good practice to match max pressure of cylinder to bottle ie 3000psi to 3000. If the cylinder is a 4500psi and filled to that, but your bottle is 3000, could be interesting (hopefully the burst disks on the bottle will work preventing over-pressurising
if you become distracted while filling). I'm not saying DON'T mismatch: if you do , just be v v careful.