I've bought stuff all over:
Brownells for OEM ruger parts, some VQ small parts (extractor, hammer, buffer), and Wolff extra power spring kits
fabsports and Jo Brooks for Kidd stuff like bolts and trigger groups (although they usually have really low stock levels

, I wish I could find a Kidd V-block)
Dlask for a DAR22 receiver and small parts like charging handles, springs, action screws, trigger group, pins, buffers, etc
Mystic Precision for a Shilen barrel
Wanstalls for a Tactical Solutions Vantage RS stock