Some of these prices are a little rich for my blood, but this label I recently found specializes in gaming OST’s from genres that are kind of all over the place. NES Ninja Gaiden, Arcade King of Fighters '98, Panzer Dragoon…
Similarly, I’ve had some luck on service 7digital. Dan Forden’s Mortal Kombat remastered soundtracks, and Alien Crush / Demon Crush OST is over there, too.
Data Discs is also on Bandcamp, and they have some great retro soundtracks too, including the Streets of Rage soundtracks and Revenge of Shinobi — I believe they worked with Yuzo Koshiro to get the original music files so they sound PRISTINE.