If I remember correctly some users offer this service, but considering your rank and reputability, it will be difficult to get a loan of testnet coin. It might be different if you offer collateral though. Take a look at the dev & technical discussion board or mining board (can't remember which one).
Or you can try regtest as mentioned above. There is no other choice since testnet coins are not free.
The whole point of Bitcoin testnet coins are them to be worth nothing so you can test with them. You shouldn't be able to sell them, I don' think that's the point.
But yeah, it probably would be hard to get that amount of testnet coins, so what I would recommend is starting your own private BTC chain which isn't actually to hard, I've done a couple times myself. You can also use the standard bitcoind node and use the regtest mode (specially for creating your own private btc chain on demand, more info here:
https://bitcoin.stackexchange.com/a/83740)
Another option would be using Litecoin public testnet or something similar to bitcoin, but this would only be useful if you only need to use basic functions like sendtoaddress, estimatefee, getblock etc etc as they are pretty much identical. (If you do choose to use the Litecoin testnet, this faucet seems to give away large amounts of coins:
https://kuttler.eu/en/bitcoin/ltc/faucet/)