Here goes nothing:
If you can find the manufacturers service manual it will tell you exactly how much refrigerant and how much oil the system contains and what kind of each. If you can not find it then there's a plate on the compressor that will tell you the model number, power consumption and hopefully refrigerant type for it. Problem with using the data on the compressor is that some compressors were made for different types of refrigerant depending on the application and a certain type of refrigerant might destroy the seals in it so it's gamble.
Most commercial A/C contractors will have the means of refilling a system; on some of them it involves desoldering / resoldering a fill port; those are definitely not a DIY job as the solder is actually a silver compound, requires MAP gas (preferably with oxygen) and even experienced users are known to mess up the job from time to time.
If the system has a fill port than you can try and find a retailer that will sell you a DIY kit for that type of port / refrigerant.
I would go with a certified / experienced contractor as the chances of messing up the compressor are significant if you do it yourself.
Most of these compressors take something else than r134 found in the car systems; most likely R22 or R404a. Also, the amount of oil depends on the design of the system and is the most common cause of failure for compressors.
