What would you need 50 testnet coins for, that 0.50 or 0.05, or 0.005 will not suffice? I don't have that much either, but you can always get from the many faucets a small amount. It's for testing, the amount should not matter.
But I'd like to also know if there are any other ways of acquiring such amount of testnet coins other than mining them.
According to me, Sometimes 0.5 tBTC does not suffice, for example: if you're designing a mixer... You'll need a lot of tBTC in order to do a proper taint analysis.