So then the person who secures your royalties did not request the orchestra books be sent. Usually you can either get full, partial, or no orchestra parts sent to you. The bass part is part of the orchestra. Maybe your theatre company should bite the bullet and get it for you, since it sounds very necessary. :O)
MTI doesn't only supply vocal books. However, the bass book for Little Shop and most shows don't have tabs. They just have the bass line. I've never encountered a bass player it was a problem for, so I don't really know what to tell you.