Are you looking for a specific one or just buying one for their beauty?
Home Depot sells reasonable sized ones for 6-7 dollars. They are a good deal but some don't have named. They just say "Pink begonia" but they are pretty. They bloom nonstop too!

Reasonable prices, maybe. I bought from Golden State Direct Bulbs last spring. It is dirt cheap if you save the tubers in the fall and use them again and again. I like the big ruffled ones the best. I have tried many times to get pictures with my point and shoot but have not had much success. In the morning not enough light and day is 100+ with sun flares and the smell of burning flesh (me).
But it gets expensive if it rains unexpectedly and half a begonia breaks off or if on marathon potting day you don't pull the drain plug on the pot. We are all just waiting out July heat. This is when all my mistakes show up.
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Here is a link that might be useful: Golden State Direct