Cannas are notorious for reproducing every year in the ground or in a pot..one year you have three,  and then next year you have ten. 

They need to be brought indoors for the winter, in their pots as is, in a cool dark place, or in vermiculite or peat in a bag or box,  not sealed.

https://homeguides.sfgate.com/propogate-canna-lilies-seeds-67371.html 

But if you crave the adventure of getting some new plants from seed, here is an article which gives specific instructions on how to do it.

Also if you have a friend who grows cannas, they will have plenty to share with you when they are preparing to put theirs away for the winter.

If you need more information online add extension or edu to your word search for university based articles.