I buy the majority of my seeds online, especially the specialty ones. These are the companies I return to and have noticed other gardeners buying from as well. You'll find good quality, non-GMO, reliably germinating seed from these places. Affiliate ...
Organic seeds bred to perform well under organic growing conditions (affiliate link)
High Mowing Organic Seeds
Good selection of seeds and there are helpful notes outside and inside each seed packet (affiliate link)
Botanical Interests
Great selection and reliable germination
Pinetree Garden Seeds
Huge selection of seeds; this company is known for quality, reliable seeds
Johnny's Selected Seeds
Great selection of unique and hard to find seeds; many have a cool backstory as well; plus, free shipping
Baker Creek Heirloom Seeds
Gorgeous seed packets; great selection; very reliable seed germination; most of their seeds come from their farm in New York
Hudson Valley Seed Company
Georgia-based seed company; lots of seeds favorable for southern US growing conditions (affiliate link)