There are several online nurseries that have them.
I have purchased several cane begonias from Accents for Home and Gardens and they were healthy little plants.

I will probably order several more begonias from one of these two places because many people on the Begonia forum here reviewed these places positively:

Taylor Greenhouses

Kartuz Greenhouses

You'll never find such a good selection of plants at a big box store as you will at online specialty nurseries. Besides, I would prefer to support independent nurseries than big box growers.

This post was edited by summersunshine on Thu, Sep 25, 14 at 22:18

I never find Begonias as houseplants except Rexes. Check with the 'annuals.'

I bought some really nice robust ones from Steves Leaves.
Much nicer plants than what I've gotten from either Taylor or Kartuz