I have gotten my seeds from several different stores: WalMart, Home Depot, and IFA (Intermountain Farmers Association). I have not found a huge difference in price, except for the American Seed Company. They tend to be really inexpensive, but when I spoke with the cashier at IFA she said they sometimes have a germination rate of 10% (only 1 in 10 seeds will sprout). In the end, this method uses so few seeds, if you store them correctly, you can use the same packet for 2 or 3 years!
If the plants you grow are not hybrids, you can harvest seeds and use them the next year. The disadvantage of this is that store-bought seeds are treated to be disease resistant.
I found this great publication by the Utah State Extension Service. It lists which specific varieties have proven to grow well in Utah. It’s possible that such information is available for other states. To find your state extension service, Google “Name of your state State Extension.”
As for plants, I bought some peppers and tomatoes at a nursery last year, and they were average. I was satisfied, but not overwhelmed by their performance. What I liked was that you had the option of buying exactly how many plants you wanted–you didn’t have to buy a 6 pack of anything. That tends to be the case at WalMart and Home Depot. However, my cousin always buys her plants at Wal-Mart, and her peppers out-perform mine every year!
Word of mouth is a great way to find good sources for plants. I made it a point last summer to notice who had great tomatoes and peppers, and asked them what variety and grower they purchased from.