Where Can I Buy Seeds for A Veggie Garden?

I was reading an interesting letter last weekend in the newspaper, and one of the suggestions made by the contributor was that we should all plant and sustain our own vegetable garden. 

I have long been a proponent of sustainable gardening and community supported agriculture, and have even tended my own bountiful garden back in the day ... but life and kids and work have gotten in the way over the last decade. So I thought for 2009 me and the mister (and the little man) would get back to our roots, so to speak, and plant a patch. A veggie patch. 

In the past we've gotten young plants from Logan Trading Co. in Raleigh, and herbs from one of the local home improvement stores. But this year, since I'm pulling out the heavy duty gardening gloves, I think it would be nice to start some vegetables from seeds. My favorites: sweet tomatoes, cucumbers, silver queen corn, zucchini, and green beans.

• Do you buy vegetable seeds? 
• Locally or from a catalog? 
• If a catalog, which ones?
• Have you ever grown watermelon? Any suggestions?