What Lija said.

Great beginner species include but are not limited to
-Fire Bellied toads
-White's Tree Frogs
-Grey Tree Frogs
-Green Tree Frogs
-Pacman Frogs
-Toads of many kinds

All of these frog species i have seen for sale.

My recommendation: dont go to a pet store. Either go to an expo near you to view the frogs before purchase, or buy online.

There are some shifty places on line though *coughs* backwater reptiles *coughs again*. The four most reputable places online are
-Josh's frogs
-Mike's phat frogs
-LLL reptile
-rainforest junkies.

Good luck !