Librarian in my opinion is pretty easy to convert as he is basically big, bald, mutated gorilla from the look of him.
And there are many of various gorillas miniatures on market.
Also I think that GW LOTR plastic trolls can be easily converted to Librarians with a bit of green stuff and cutting and filing of some excessive bits.
Metro 2033 Collectors edition included a couple of figures one of them being a Demon and you can still find these figures on Ebay or assorted Russian internet flea markets for a much more reasonable price. I have this figure in my collection and can say that he fits rather well with 28mm minis.
https://www.ebay.com/itm/Metro-Last-Light-Russian-Collectors-Edition/263886896930?hash=item3d70e27f22:g:IBQAAOSwtjI had no success trying to find convincingly looking watchmen minis and it would be very interesting to see if someone can suggest something fitting in this regard.
Also 3d models from the game are available on various sites on the net and perhaps it would be easier to find someone with a bit of 3d printing expertise and try to print them to scale adding greenstuffed details to the minis.