What about mining them?
Absurdly, mining testnet bitcoins is really expensive. Currently, it has a difficulty of 61681740.29071233 which means it has a target of:
0x0000000000000045a14a00000000000000000000000000000000000000000000
Your odds of generating such hash are 0.00000000000000037% or 1 in 264,925,099,745,189,347.
If you want some tBTC to make testings, I'm running a forum faucet:
[Merit] [Faucet] Hey Bitcoiners! Wanna try out the lightning network?