It must be cheap.
Some will recommend decentralized platforms like bisq or so, I have no experience with those.
But if you prefer centralized exchanges, you can DCA on those too, just make sure you don't withdraw too often (withdraw only after your buys have passed a certain threshold the withdrawal fee no longer hurts s much).
So, we still need to be careful if who we are dealing with. There are more famous centralized exchange but the only problem with them is they are in control of your funds. We will be storing our funds on them for a long time, if dca'ing is our strategy since we are only buying in smaller quantities and withdrawing them in an instant is not wise.