BET-C is a corrupted MULE in Deep Rock Galactic, and acts as a miniboss that can spawn in any mission in any planetary region in the game. The spawn rate is 3% for Hazard 1, and 6 % in all the other Hazard levels.
Finding BET-C has more to do with luck than anything else since it will randomly spawn as a boss. So just play a lot of missions and you will eventually find BET-C. It is suggested to play on Hazard 2 or higher to increase the spawn chace.
Once you find the BET-C, you have to fight and defeat it. Once you defeat BET-C it will be "fixed", and you get the trophy / achievement called "Now We Have A BET-C!".