You can try to email dutchgardens.com. Their tuberous begonias were nice and they shipped early to me.
If you really want nice sized begonias, visit antnelli.infopoint.com (antonelli's begonias). They have some very very nice begonias. Try their start "Starter Assortment of Begonias - Special Dozen Upright Tuberous Begonia". You will not be disappointed. If you call them or email them, they should be able to ship earlier if you agree to UPS 2nd day (they did this for me).
Finally, go visit walmart. They had tuberous begonias this week in our area.
The begonia pictures below are from the Antonelli's Starter assortment.
