Well, in the more "free" arena, local friends/neighbors (I know I'd be thrilled to share seeds with a neighbor interested in gardening!), local garden club, wintersown.org, seed swaps here and on other websites.
As for companies.
I only buy open-pollinated heirlooms - so I can save seed from them. I'm weird like that. My fav companies so far are...
Irish Eyes Garden City Seeds (they specialize in shorter-season seeds - my local grocery store carries them as well, and I have yet to be disappointed in the 3 years I've bought from them)
Baker Creek Heirloom Seeds (they put a ton of seeds in their packets, shipping is nominal)
Territorial (again, shorter-season seeds to some degree)
Nichols (yup, you guessed it, a decent shorter-season seed selection)
Heirloom Acres Seeds (family run thing, slow shipping where it takes 2-3 weeks for them to package the orders, but the costs are among the lowest I've seen and the seeds have done well for me, and last year I got a mislabeled variety and Thom sent me a packet of what I *should* have gotten and another freebie of another variety - haven't been disappointed in the seeds in the few years I've ordered from 'em, and I order in January to make sure I get my stuff before I need to start seeds in Feb/Mar).
I've also ordered from Bountiful Gardens and... some other place I can't remember right now.