Some isoquionlines are in cacti. I believe Alexander Shulgin has a book titled Simple Plant Isoquinolines, though i haven't read it and don't know how many are active, and of the active ones, if any are useful.
IIRC tetrahydroisoquinoline is part of the structure of some muscle relaxers.
EDIT: Some are also toxic, i wouldn't go ingesting any on a whim as I know next to nothing about them