Most Aircon places work on a trade in system where you trade in your old compressor for a "new" (reconditioned) one.
The guys I know are Auto Aircon (Silverton Radiators) in Eastleigh, Edenvale, GP. They are the biggest sellers of trade in units.
I would advise you to take your car to them cos a compressor can die in different ways which could cause consequential losses.
If the seals go and you get oil in the rest of the system you could be in for an expensive ride.
Auto Aircon are the cheapest that ive encountered but still not extremely affordable.Aircons are not cheap by any means.