Yes, OkCupid is free. There is an A-List premium feature that gets you some added perks, but it's completely optional. Without it, you still get full functionality - meaning you can fill out your profile, upload up to 10 pictures, search people effectively, and message back and forth without limits.
FetLife is also free, and it also has a premium feature that you don't have to pay for as well.
I feel your pain, by the way. I'm in Tulsa, Oklahoma area (kind've north eastern OK) and while I have found our poly groups on FB and Fet out here, as well as encountering some Poly people/couples on OkCupid, I haven't had anything work out. I've come close a couple times, but in the end, nothing. Either there isn't interest on one side or the other, or they're too far away, or they're not really looking.
So I have myself out there on these sites, and Tinder/Bumble as well (every once in awhile I see someone say they're poly on those), but yes it's frustrating. So many awesome people that just aren't poly and aren't open to it and it can feel like being poly is like shooting yourself in the foot because you end up being limited to such a small pool of potential people...
But hopefully we'll find someone(s) eventually. I suppose it's inevitable if we just keep at it long enough.