Check out your local garden shops for tomatoes. the local garden shops are starting to carry some of the heirloom varieties.

While there, ask one of the more informed staff about other vegetables. Frequently some shops will carry a couple of interesting things each year.

Most veggies are annuals, and quite fragile when young--I don't think they would ship very well, although that's not to say someone doesn't.

The veggies that get direct sown from seed usually don't do much better than more develped ones planted prematurely--try putting in some squash, cukes, and bean seed in early June.

You could be planting pea seeds now, as well as any of the cole crops; they all handle frost well, as do lettuce, radishes, beets, and spinach. Most of these actually do better in cooler weather.

I buy tomatoes, peppers, & eggplants at Cross Country Nurseries. They have a huge selection, and many of their offerings are heirlooms or open-pollinated types. I've not done mail order though; they're a short drive from me so I buy my plants directly. I do know they ship all over....and that people drive from all over to visit them.

This year, I also picked up some basils. And, in addition to the things I mentioned, they have tomatillos and a selection of cilantros as well.

http://www.chileplants.com/