My favorite begonia supplier is Lauray of Salisbury. She also sells cuttings at half the catalog price, which I like a lot because I get more plants. Rob's Violets also sells begonias and is very reliable. Mostly theirs are smaller-growing ones. Of course, the absolute best source is begonia conventions! There are other sources, but they seem to have mixed reviews or I haven't ordered from them and don't know.

Palm Hammock, Kartuz, Glasshouse Works all have a good selection but not of rexes. I've ordered from all of them plus the two greenelbows mentioned and Logee's as well. They all have their good and bad points (price, substitution policy, delivery speed, selection, etc.)

Lyndon Lyon has an array of rexes (I have not ordered from them). eBay is hit and miss and is best to buy more than one from the same seller to minimize shipping. There are several nice ones being offered now but shipping can be higher in the winter (I think they make more money this way).

Speaking of begonia conventions, the national one is in Miami first week of March this year. Check Palm Hammock Orchid Estate's web site for some details.