You can buy MYST on one a centralised exchange where MYST is traded, such as MEXC, and then withdraw it to your wallet, such as Metamask, to trade it further.
You can find Ethereum MYST in the Uniswap V3 pool, and bridge these through the Multichain V3 router. However, this option may not be the most cheapest.
You can buy BSC MYST on Pancakeswap exchange and then bridge them through the Multichain V3 router.
If you already have funds on the Polygon network, you can use them to buy MYST on the Quickswap exchange.
Find the full list of Mysterium markets here: https://www.coingecko.com/en/coins/mysterium