It is frustrating when one can't get the books they're looking for (especially if it's a series and part of it's out there).
That series is old enough [90's] that it's possible the rights don't reside with a publisher (the last one is new enough [2010] that it might be the only one the publisher has the rights to), but with the author himself. Might be worth emailing him to see if he plans to self-pub them or if he doesn't hold the rights he might know if the rights holder has any plans to put them out.
He doesn't have an email address out there, but here's his agent's address (the agent might have a better idea of things anyway)...
christopher@selectricartists.com