It's like saying "where do I buy a car? " Some people don't realize that high pressure compressors can range from expensive to exhorbitant, kinda like diesel engines to which they are similar in some ways. They may also be unaware that some really cheap compressors are available on EBay but also not understand that they may have problems or the particular offering may be a scam. Some of the cheap compressors are military surplus and will only pump to 2250 or 3000 psi and may require mods or repairs. So, do you want a "renta-wreck" or go with Diamond Joe Brady (Bauer)? There are two Cornelius compressors on EBay right now. One of them is dubbed FreeAire. The other is unconverted military surplus. There is also this Airetex Co (the curley one) who sells Turkish compressors (Alkin). They are similar to Bauer but cheaper. However, after the sale, your day to day ability to consult with the company may vary. He's out to lunch or DEMA, or whatever. So, when you decide to ask us a serious question how about giving as much as you get: How many tanks, what pressure, what connections on the tanks, how mechanical are you, do you have 220v service, can you run a gas engine, can you be in attendance for two hours while your tanks fill, how much money you got-- for starters.
Disclosure: my dive locker compressor is a Bauer, my portable is a home assembled engine driven unit based on a military machine. It has the same performance as a Bauer Oceanus but weighs 10 lbs less and cost 1500 bucks plus a lot of work. I "love" the old Bauer Capitano but wouldn't trade the portable for an Oceanus, not even close.