I agree with Cheri.

The ten gallon that's housing my medium White's Tree Frog right now (I know it's small, but I'm in the process of building him a new home as I type this! ><) with be converted and used to house some thumbs. Granted this won't be until October or November, but it's always best to plan ahead.

I researched conversion kits a while back and they're inexpensive (like Cheri said, $20) and the tank ends up looking completely professional. They're also very easy to install by the look of it. Just need some silicone and you're ready and set.

Since ten gallons aren't a lot of space to work with it's best to try and find the smallest frogs you can so they'll be comfortable in their environment. I know some people who keeps pairs of thumbs in tens without any problems, as long as it's properly planted and constructed the frogs should be happy and feel safe.

Good luck in your search! I hope you find what you're looking for.