Please do a search in mostly the Music Electronics section at the top of the menu. We have had MANY MANY discussions on running a shop, and central to that is buying and sourcing parts.
You will NEVER eliminate the need to order parts. Having a reasonable parts collection on hand depends on what your reasonable repair load might be. You just gonna fix jacks and pots on cheap amps? You gonna "only work on tube amps"? Gonna stick to fender? Gonna take in PA and recording gear too or only guitar amps? You want to cover 75%, 80%, 90% 99% of potential repairs? etc etc.