you cannot really buy at the moment because the token has not been released officially.
anything you buy now is really an IOU and you are trusting the exchange to give you the real token when it does finally launch...that is expected in 2020 Q1/Q2
anyway if you want to buy something now then use coinmarketcap to check,

https://coinmarketcap.com/currencies/filecoin/markets/reported/

gate.io and lbank are good exchanges, never used bitforex