Twos- so far completely reliable sources VTI Gunparts (google) and Cimarron Arms Co. In the rare event one doens't have the part the other always has.
Buy hand assemblies- hand and spring, trigger-bolt springs and mainspring. The hand spring is the one I've had to replace on a couple of guns and the trigger/bolt spring is also prone to metal fatigue and breakage.
the hand spring is difficult to attach to the hand so, they generally sell the smaller ones- navy, army, pocket model as units.