If your a NA resident my best advice if you want the OST is to try and get to a convention that has Anime or Manga of some form. It's no guarantee but to date if its a game or anime with a soundtrack i can find it at one
personally not knowing your locale i always go to Anime North and Fan Expo in Toronto and can usually find it.
The other option i can recommend is if you have a Manga/Anime store near your location that sells even some CDs give them a look and they might have it, whats more if you can find one in an Asian District (sorry if this sounds racist i don't mean to offend) but there is a store I frequent in the one district a little ways from here that always carries this kind of stuff as well as just about every other import from Japan you could ever want, except for figurines T_T. Worse case scenario they might be able to order it for you as well
Lastly, ya just try checking digital download sites, or import sites. I dont generally frequent them all that much since i enjoy hunting for stuff like that but not a bad way to go :3