I used to have a favorite "S Scale Hobby Shop" but have been very disappointed with them on the last couple of orders (items listed as "in stock" not actually in stock, not refunding payment on out of stock items until "reminded," etc.)
I'm not naming names because in the past they've always offered great service and I'm hoping my last couple transactions were just glitches. For now though I'd recommend going straight to the manufacturers. Most are happy to sell direct.
Cheers,
Ken