Besides Squadron, for plastic models I also use Sprue Brothers, Brookhurst Hobbies and Hobby Link Japan, depending on what I'm ordering.
There are some manufacturers that you can order from directly, like Eduard.
I also use Model Expo for wooden ship supplies. Micro Mark is a good souce for specialty hobby tools and supplies.
Finally, I like to build older "classic" kits, so I shop a lot on Ebay, looking for deals.
HTH.
Mark