I'm doing a self-study course for the New Product Development Professional (NPDP) certification. There is no test prep material like there is for the PMP test. Fortunately, there's a reading list that's not too extensive and the books and articles are easily available if you have access to a good library. Many of the books and articles are real classics in the field.
Some people think certifications are a waste of time, but I've gotten a lot out of studying for them, and then trying to apply the knowledge.