You can check the seed exchange forum or Karchesky Cannas has quite a few of their seeds for sale usually. You should know that they don't come true from seed although some of them will look the same as the parent.

Here is a link that might be useful: Karchesky Cannas

I've got plenty of varieties of canna seeds that I will share if you'd like to pay postage for them. Like Jaceysgranny says, not all of them will come true from seed, but you might get something even better than the parent in some cases. Email me through my member's page if you're interested.