I'm not american and don't know what you normally buy over there.. but I've done alot of car audio installs, and the best brands on that list are definately:

Orion
Power acoustik
Phoenix audio
Pioneer
Rockford Fostgate

If you're making a car stereo store, installation equipment should matter too, i'd recommend for starters to get some cheap awg0 wire, signal cables, speaker wires and fuse boxes to sell as package deals, that usually works very well.

You should definately have Alpine headunits in stock, as they're maybe the most popular brand in your customer base. Pioneer headunits are also essential. For each expensive brand you should have an eco one to counter it aswell. But if you're wondering about which highend brands to carry, i really recommend genesis, audison (for amps) and hertz (speakers and subs)