I am going to put in a shameless plug for Baker’s Creek Seeds for Heirlooms, they are headquartered in Missouri around my hometown area.
I have also had good luck with Bountiful Gardens, Southern Exposure Seed Exchange, and Jung’s.
I just got my order from Baker’s Creek, and I already have my wish list for next year!
And here’s my plug for Pinetree in Maine, and Sample Seeds in upstate NY - inexpensive, great selection and good resources for the northern gardener.