The function of the companion plant is to tell the season of the bonsai display. So, you might be looking for crocus or daffodil in early spring, dry grasses in winter, etc. Whatever you can find in your local nursery or on a trail hike would probably be perfectly appropriate.
All kinds of herbs have interesting leaves, and you might also see if there some type of exotic or unusual nursery in your area. Many companion plants can be grown from seed, too!
black mondo grass and dwarf mondo grass make interesting companion plantings. There is a nice article by Kevin Bailey if you click
bonsaiTALK Links and enter ACCENT, as well as the Wolfgang Putz's gallery of hostas and other accent plants.
Regards,
Matt