I got them from the local nursery. but I also found out that you can lift the tubers at the end of the season, and store them like gladiolas for next year-

I lifted the chartreuse one, because it had totally out-grown the bed I had it in, and got a dozen tubers (and I only really need three of them) and if you'd like, I can lift my purple one (it is getting to be that time of year) and see if I have a few I can send you...

they can get hung up in a mesh bag any where cool, dark, and dry. dusting them with baking soda will help prevent anything fungal from taking hold, and replanted with the tender annuals.