I've seen packets of cactus flowering talls at Home Depot. Could check to see if they still have them in the seed racks. Also, in the past, I've seen packets of the tall dahlias at Target -- another possibility.

We have 2 or 3 cactus varieties that came from seed packets we got at home depot. Most seed packets you see in seed racks are usually dwarf types and they do well. I have a few beds of dwarfs but they don't give the blooms that the larger varieties do. Local florist like large blooms and they want long stems. We breed some and buy seed to expand our gardens.
I was hoping some breeders would connect with me and we could
exchange seed or I could buy some. It's not easy to breed dahlias a lot work is involved.