It depends on the subject.
Ships- Freetime Hobbies and White Ensign Hobbies.
Photo Etch- straight to the source Gold Medal Models and Toms Model.
I like Squadron, but their website is a little hard. It seems straightforward, but with some clever tricks there are a lot of items that you can find with a little subterfuge. not a great thing as a retailer.
I buy a lot of stuff like paint from Amazon. If its there its in stock, and usually here within a week.
They are my favorite.