The bigger question is what you are going to do with 3000 psig compressed air. If the compressor is not made to produce oil-free air , no recharging Scuba tanks or self-contained breathing apparatus tanks for your local volunteer fire companies. Oil-free air delivery from piston type compressors was accomplished by way of "Teflon" piston rings, and/or a series of filter traps to remove any entrained oil in the air. If the compressor was never built to deliver oil free air, chances are it may have been used on high voltage air circuit breakers. High pressure air was used to both operate the breaker mechanism, as well as to 'bend the arc' formed when the breaker contacts open.
There are some aftermarket service shops which supported the older high pressure air compressors used on air circuit breakers. Possibly, a search along that line might turn up an aftermarket supplier. A firm I've had good success with over the years for large rotary screw compressor repair and retrofit work is Air Compressor Technologies in New Hampshire. Gene Specyalski is the owner/guru. He knows compressors including some of the recip compressors. Possibly Gene can put you onto a source for a manual for your high pressure compressor.
From what I remember of the high pressure air compressors we had at the powerplant, they were 'interesting'. We had some Bauer (Swiss made, originally on Brown Boveri air circuit breakers), and we had some Worthington (supported by a high voltage breaker service firm called Harley, in Twinsburg, OH- go figure that a firm named Harley will be in a town with the word "twin'). I remember that on the high pressure or final stages, the pistons on these compressors was quite small, maybe 1" in diameter. I also remember that a larger diameter cylinder jug was used as a crosshead guide for the HP piston, with the HP cylinder 'tandemed' onto the head end of the larger cylinder jug. The HP compressors were quite finicky to work on. Eventually, we upgraded to different types of circuit breakers, so the HP air compressors were phased out. I think one of the larger uses of these types of recip high pressure compressors is likely for air circuit breakers, though other industrial equipment or processes also might require high pressure compressed air.