Over the years I have purchased my garden seeds from a number of different places. I have ordered seeds and I have purchased them over the counter. If you order seeds you need to do it well in advance to be ready when the ground is ready to work in the spring. Nothing worse than ordering seeds and not have them when things are ready in the spring.
When you order seeds you can get just exactly what you want. If you prefer a certain variety of bean or corn you can pretty much get it some where. You can also try varieties that you would never see in a store. One place I have ordered from a few times is Johnny's Selected Seeds in Maine. When I first ordered there years ago they just had a little black and white catalog. Now they have a nice website to order from. They have a lot of varieties that are very interesting to try. We have also purchased stuff from Henry Fields Catalog. Not sure if they are still in business even.
If you buy over the counter you are pretty much limited to what the retailer has in stock. I used to buy seeds at the local hardware store. They sold Burpee seeds which I liked but they were not a big store and had a limited choice of varieties. It was about the same with any store in the area. The hardware store is now closed, the owner retired. There is a new lawn and garden store in town and they have a small selection of stuff but not much. They did order some of the Royalty Purple Pod green beans I like but they sold them all before I got in there to buy some.
The last couple of years I have purchased seeds at a natural foods grocery store where we shop. They are just small packets but it actually suits my garden philosophy of smaller individual plantings and more variety. They sell seeds by a company called Botanical Interests and they are all Organically grown seeds I think. I have had good results with them. I also purchase some Ferry-Morse Organic seeds at Lowe's. I planted some of their Okra this year and I planted a few things last year and had good results.
I usually buy organic seeds when I can. But, the sweetcorn I plant is usually treated seeds. It is hard to find sweetcorn that is not treated around here. I do not buy the small packets of corn since it would take too many so I buy my corn in bulk at the local garden store. I might order some untreated seeds next year if I can get it done early. I like to plant the early varieties of corn. We had our first sweet corn this year during the last week of June. I was a little delayed by wet weather and cool temps in the spring planting but good rains made up for the late planting.
I usually purchase my green bean seeds at the garden store but this year I planted some of the small packets from Botanical Interests. Each packet had enough for about 10 to 12 feet of row and I split my rows between a couple of different varieties, half yellow was and half green beans.
I just planted a green manure crop of buckwheat on a small section of the garden which has germinated and is coming up pretty good. I purchased that seed at the natural food store. It was food grade buckwheat that was unhulled. I planted it right a day or so after we had about a half inch rain and then last week we had about two inches of rain over a 12 hour period so it came up pretty good. I will let it grow until it flowers and then till it under.