One of the keys to becoming sustainable is by saving your seed from year to year. In order to save seed one must use heirlooms seeds. Now i will be the first one to tell you that trading with ppl is the best way to obtain seeds,its cheap and fun but sometimes we must purchase them.
Not all companies are created the same. Some do not do any testing on the seed while others do. I personally try and use companies that do test the seed for adulterations (gmo tainting amongst other things) . That however is ultimately up to you the buyer to check each company out.
Here are a few companies that i like to purchase from .
the best herb seed store i have ever seen