I'm using Horn Canna Farm this year. I've read nothing but good things about them.

Horn is the one I started with to get a lot of the common ones reasonable price and they were good size and quality.

The Horn people are the BEST! My yard is full of exotic looking cannas they sent me over the years. They are also very generous with their corms and these are fine quality and absolutely huge.

Also try Karcheskycanna.com They have beautiful cannas and also sell seeds if you take a notion to try that. They have some beautiful and exotic varieties. Check them out!