They're not rare... where I live, they're sold in most chain pet stores.
I'm not too sure about the dwarf frog, but it's larger cousin, the african clawed frog, is illegal in some states like California and Florida due to the amount that have been released in the wild and are now thriving, choking local ecosystems, etc.
Personally, I try not to buy animals online, but if its your only hope and you don't mind paying an extra $20 or so, check out this link:
http://www.****************/catalog_pages/misc_critters/critters.htm#Frogs
Oh, be sure that the frog is legal where you live. I really don't think the dwarf frog has any legal restrictions, but it could.