Some of my favorite heirloom seed companies. Don't forget to research heirlooms from other countries, sometimes you are surprised as to what is out there. I will add more as I get time. Hope they help.
Seed Savers.Org
https://www.seedsavers.org
Pepper Joe's
https://pepperjoe.com
Southern Exposure seed Exchange
http://www.southernexposure.com