Good Compressor

[U.S. Water Rockets1]

Here's the 4500psi compressor boosterthat was designed to be fed air from a normal 125psi compressor. Says 8 cubic ft per hour fill rate.

Guess it would take years of use to pay for itself unless you take up another hobby that requires high air pressure.

Bugwubber

8 cubic feet per hour means it would practically takes years of use to fill a water rocket.