I'm building an app and I'm working with btc testnet coins, but I want to be able to convert them to other altcoin testnet coins. Is there such a service out there?
Most testnet coins have a working faucet to obtain coins. Alternatively you can usually mine them with a CPU.
There isn't really a service that can do that as there isn't a purpose but you can, however, is mentioned already are mine or use a faucet or ask anyone you know who has extra that you can use and after you finished with it, return or give those testnet coins away to other developers that need it.