For regular cars and small trucks, 1/43 is what I have been using. For some of the larger vehicles I have bought 1/48 and 1/50... Variety is good, and standard sources are flea market, garage sales, thrift stores, and bargain/close out shops. And eBay of course.
Anything diecast is suitable, though I prefer models with doors that open, and don't have those large bulging under carriages (where they house the windup/ spring mechanism)