I also like Tomato Grower Supply. Their office/shipping is located here in Florida. They have a mix of hybrid and heirloom seeds. Previously, they carried mostly tomato, peppers, eggplants, tomatillos, but they are expanding into squash, pumpkins, broccoli, etc. I have been happy with their products and service.
I like Baker Creek too but have had some inconsistency with their seeds this year. My usual cow peas aren't exactly like they were a few years ago and two different squashes had a few renegade seeds in them...there would be one or two odd squash in the row that was close but not exactly like the others. They source some of their seeds from customers and I think there might be a little cross pollination involved. I'm not really complaining but I'm going to start saving my own seed to keep it true. I so wish I'd done that with the peas but I got ahold of a great strain that came locally so I'm switching to them completely next year.
I'll still love Baker and will continue to buy from them but when I find something I just really love, I'll be saving my own.
I have gotten from Baker Creek, southern exposure, victory seeds and seed savers exchange. I just ordered from a new source this past month for the fall, it's called two seeds in a pod out if West Virginia. Will give an update later this year