Yes, you can get those small bedding sorts of dahlias here too. There are some very nice ones available. But you had better be prepared to snail bait and/or snail patrol like crazy.
My problem with dahlias is that our soil is really dense clay, so I've lost dahlias to winter rot or something. At the moment, I have one surviving dahlia which is just beginning to come above ground - 'Bishop of Leicester'.
I don't want to grow dahlias in pots - I'd need huge pots, and I have enough pots to manage as it is.
For late September into October I like Chrysanthemums. Unfortunately, I discovered that the rabbits like them too; they ate most of those that I had in the ground.
(I still have one small white-flowered one in the Moon Garden (which might be 'Snowtime)'; I don't know why they didn't find (and eat) that one.
) So now I am forced to grow them in pots (both the large "florist" kinds and even the smaller NOID ones you can get at your local nursery or hardware store garden center). During my brief time being a member of the local Chrysanthemum Society/club (brief because our interests were not the same - they are about shows, I am about color for my garden), I picked up a bit of information on growing the larger mums. Right now I have 3 survivors from last fall, 9 purchased cuttings I just potted up, and come the local club sale (early June?) I will pick up a few more cuttings. That is in addition to the small Moon Garden plant, and two surviving NOIDs from the hardware store. Growing them in pots is a pain, but at least they give me fall color.