There's still liqui.io but with much lower volume, and Upbit having "Exclusive partnership with Bittrex", so not sure MYST will stay there. Anyone knows...?
Upbit mirrors the trading activity from Bittrex and runs its orders in parallel. From my observations, the Upbit volume may be 5-30% higher than Bittrex's for the same pair.
However, Upbit has additional KRW trading pairs for established altcoins.
After Bittrex delisting, which I hope is temporary, Mysterium will not be available for trading on Upbit.