There are a few great sources for buying seed, and a few I steer clear from. Last year, I posted something on seed ownership, and it's a great link to check out. Seeds of Change is a west coast company with testing fields in Oregon and beyond (I believe, but don't quote me on it!) and they have an awesome website with lots of great information. You still have to do your homework, as even though they are located in the Pacific NW, they sell varieties that won't do well here. Pay attention to the growing cycle of eat plant and make sure they don't require a long hot season is you're in a Maritime climate. If, like my family, you're in hot hot NY - plant some corn!
Seed Savers Exchange - oh my god, these folks have the bomb seeds. They are more costly than others, for sure. That kills me a little but, but it's a great great organization dedicated to saving and sharing rare and heirloom seeds. I just picked up a packet of "Crisp Mint Lettuce" when I was at Kettle Falls Meyers Market (as if I needed more seeds) last week for a book signing. It's a Romaine like lettuce with frilly leaves. Can't wait.
Baker Creek Heirloom Seeds - this company is totally rad, but they carry a lot of hot varieities that would just not do well in the Pacific NW. Check them out for inspiration and rare seeds. They are small and wonderful.
Osborne Seeds - a local seed company that sells varieities just right for our climate. Sorry, east coasters, but HURRAH for us!