Board Thread:Questions and Answers/@comment-79.232.87.228-20151020170657/@comment-917693-20151030212711

Hellos~ Usually bonus discs are from limited editions of albums. And according to UtaiteDB http://utaitedb.net/Al/6142 the bonus disc was event limited (obtainable at the Comiket it was sold at).

I suppose there may be people who sell the bonus disc second hand, but other than that, I don't think it's purchasable anymore.