>>I've got a White's Tree Frog and I'm looking to purchase another frog that can co-exist with it. I'm not interested in another White's as I'm looking for some variation.>>

Practically any treefrog that is about the same size will be compatible. Treefrogs usually do not eat other frogs, except when they make a mistake thinking that the thing that is moving in front of them is an insect. Don't mix most frogs with firebelly toads because these can release toxins into the water and poison other frogs.