A few tour operators in Cahuita and Puerto Viejo de Talamanca can set you up with shuttle transportation to Bocas del Toro. They also offer day tours to the islands, but travel time and border formalities eat up a huge chunk of that day. Bocas is a fun destination in its own right and worth spending a couple of nights.
For more information about Bocas del Toro and Panama than we can supply here, pick up a copy of Fodor’s In Focus Panama.