I will reiterate White Flower Farm. They're in zone 4, which is nice and cold.
I buy my culinary herbs from The Tasteful Garden (
http://www.tastefulgarden.com/index.asp). They grow well and taste wonderful.
I buy tomato and pepper plants from Territorial Seed (as well as potato starts and lettuce seeds)
http://www.territorial-seed.com/stores/1/index.cfm