How to buy the polkadot version

I really want to buy some efinity. Their white page states it is listed on the polkadot network. 10% of my portfolio is polkadot but I've never attempted to buy a token on their chain. What dex is best for doing this? I see I can buy efinity as an erc20 token but Ik this can't be the same as the polkadot token. Also, $25 transaction fee is too rich for my blood. Help please