Oh how lovely they are! I am not a wholesaler, so don't know how to get them...particularly the 'New York Swirl' and the "Charlotte Chiffon".
There are two places that I know of that have GAC - one is on eBay (Andiel's garden and other occasional sellers) and the other is Plants for Kids (I've bought from them a couple of times and they ship fairly quick). I just did a quick check on Andiel's and I guess he quit eBay or maybe it's the time of year.
Logees has a couple once in a while so check them out. Local stores carry them at times and hopefully they will become more common place in the future. I bought 'Taurus' at a Home Depot in California back in September. They had several GAC begonias as well.
Anyway, check PFK. The more you order the cheaper they are. They went up on prices last year.
Booman Floral has them as well but I think it is wholesale only.
You can find begonias at glasshouseworks.com
they have many varieties.