Locally check garden centers, nurseries, and Home Depot, Lowes, and Wal-Mart. Also see if there is a local chapter of the begonia society close to you.

eBay is another place to try. There are some good growers (some not so good) there.

For on-line there are several places that have a large selection to choose from. Some are excellent, some are slow, some are on the high side, and some that ship the wrong things, etc.

Palm Hammock
Logee's
Kartuz

Rob's Violets
Glasshouse Works
Lauray

There are several other on-line retailers too, but have a much smaller selection to pick from.

Butch