Prairie Moon has a nice selection of natives. Also Prairie Nursery. They are both east of the Mississippi, though. You might have to ask other folks about California natives. Of course, any major seed company will do for cultivated plants like zinnia.
Liz

I second Dr Liz's recommendations.

Martha

Actually, lots of seed-getting occurs via the seed exchange forum, and it's really quite fun. You can find a surprising diversity. Though, it's harder if you are just starting out and don't have much to trade with. You can search for individual seeds you would like, and you'll get a list of the Gardenweb members who have that seed listed for trade. Then you can contact them to see what kind of exchange you might set up.

Good luck.

Martha

I've purchased seeds from the following companies: Prairie Moon nursery, Prairie nursery, Everwilde, Swallowtail, Diane's, Parks, Ebay, Easywildflowers.com, Lily Auction, and probably some others I've forgotten. Plus I buy packets in stores and have used the Seed Exchange and other forums to do many trades, a few SASBEs and shared many seeds to others.

Most seeds do well, but occasionally some won't germinate, turn out to be label incorrectly, turn out the wrong color, etc. The quality control is better with commercial seeds, but they have lousy seeds and occasionally make mistakes too. It's all fun though!

I like Terrene's ideas...I would throw in amazon.com too. when buying from vendors like ebay/amazon make sure the seller has excellent ratings, and your odds are even better if they specialize in seeds.

the seed exchange is more of a gamble...you're more likely to get the wrong seeds or non viable seeds, but i've definitely had more good experiences than bad.

I just got some viridis milkweed seeds from everwilde farms. Hope they survive our Minnesota winter :)

Another possibility is Rose Franklin's perennials.

Here is a link that might be useful: Rose Franklin's perennials

Thank you I'm going to try Ever wilde Farms.

Thanks, Nicholas