I've been through this myself, had the compressor sounding very 'grumbly' when activated, try not to use it because it could seize and throw the belt off.
Usually botched regass merchants don't add enough (or any) PAG Oil to the system when servicing and this wears the compressor - when mine was going it also stressed the Aux Drive Belt to the point where it was very cracked through overheating.
Sourcing a replacement is really a case of matching up the serial numbers etc, as far as I know Volvo used various types - mine was a 'Zexel' unit with FoMoCo stamps all over it
