Try Ebay. The prices can't be beat, and for the small amounts of money you will spend, if a few rhizomes don't grow, then no big deal. I have bought every single one of my cannas off Ebay, and have only had a few not produce stalks. I would guess that that was more my fault than the seller's.

For my Stuttgart and Pink Sunburst. I brought them from http://www.thewatergardenshop.com/marginal.html They send you a growing plant so you know what your are getting for 9.99.

I got mine from a very recommended source last year. I was
disappointed because many were infected by virus. I don't
know what I am going to do next year. Maybe I won't grow
them.

If the actual variety or cultivar isn't too important, post a "wanted" on your local freecycle or craigslist, maybe mentioning the colors you want.

*Somebody* will always need to thin their cannas!