When the soundtracks are released, they're slated to remain in production for two years, so after that they join the unfathomably large ranks of the out-of-print discs. So just think how old any game is, taking its month of release into account, and you can easily determine which ones are still available. As for special discs though, like THE KING OF FIGHTERS best arrange collection ~since94to00~ you just have to know about them. Fatal Fury & KoF soundtracks seem to be the most common ones of all, not quite surprisingly.