Autobacs in Stanton, CA is a great huge store to buy all kinds of different car parts; especially nice Japanese parts.
If you're looking to upgrade some kind of part on your car with a high quality product, then this store should be on your shopping list. The prices are kind of steep, but also consider they have things that you might not find else where.
If you're a Japanese car enthusiast, this is a cool place to just go and window shop. They have installation bays and installers in-store and you can watch them do their work.