there are many great organic and heirloom seeds places out there. It's great to grow heirlooms because you are helping to save the variety, plus many of them have superb flavor! My favorites are Botanical Interests, Renee's Garden, Seed Savers, and Cherry Gal.... but there are plenty of others. I highly suggest all of those. They have more variety than you could ever use!
I use parks and have not had a problem except for stuff being on back order for about 2 months now. I like ordering off the internet because you have more choices on the seeds and they are fresher then the ones you get in the stores (I guess)
I guess it depends what you are looking for I have used baker creek and i recently have used www.ohioheirloomseeds.com I like the heirloom plants so i go for the heirloom companies. I have also bought plants (native herbs) off of ebay that i couldnt find elsewhere.