As I explore and use sources for parts and supplies, I’ll list them here, with an emphasis on N scale material.
It should come as no surprise that Amazon.com has a large inventory of items useful to hobbyists. On commodity items, their prices and fast shipping are hard to beat. You will find products from the bigger vendors here, such as Woodland Scenics, Digitrax and so on. Good DCC product coverage overall. More scale specific coverage is spotty at best, with most being from 3rd party vendors. If it isn’t a common sort of item, you probably won’t find it there.
The good news is that other internet vendors of scale specific hobby gear are very competitive and offer some good values and service.