Never visited the Mountains? Besides harpies, you can meet in the wild: Dragonkins (1), Seraphs (2.5), Gnomes (3), Centaurs (2), Goblins (4) and Orcs (8).
Those between brackets are the weighted chances: they tell you how likely a given race is to be met, relative to all other races (e.g. for one dragonkin, you meet eight orcs).
2.5 / (1+2.5+3+2+4+8) = 2.5 / (20.5)≈ 12.2% of monstergirls are seraphs.
Chances of meeting any monstergirl are 2/12, so the absolute chance of finding a wild seraph is only (2/12) * (2.5/20.5) ≈ 2%.
In the deep Mountains the odds are actually a bit higher, because monstergirls sometimes travel as a couple.
Slavers from the Mountains may sell Seraphs too (along with Dragonkins and other races from that region).