I don't believe FFG is providing the mini's unpainted separate from the game. HOWEVER, I think you WILL be able to find BGG members interested in selling their MONSTER mini's.
The monster miniatures take up too much table space, amid a host of other issues (the bases don't work well, etc). A few players have mentioned that they don't use the monster mini's, they just leave them in the box. Instead, they use the monster cardboard token on the board (which have relevant gameplay info, they are similar to the monster tokens from Arkham/Eldritch Horror).
So I think there will be a market for those looking for the MONSTER mini's. However, the hero mini's are another story. They don't have a cardboard equivalent in the box. They will be harder to come by.
Was checking to see if the minis was possible to buy in 1s and groups. Not much luck when truth be told. Seems like it could be easier and in fact cheaper to just go buy the entire game packages instead of buying them as singles. Mostly found were $7, $8-$10+ for each single miniature.