If your not farmiliar with the boat the best place to get your parts will be a marine shop. They probably have the manual in stock and have the expertise to answer any stupid questions you might have... I know they have answered some stupid questions for me in the past
If you know exactly what your looking for there are several online stores that sell Mercruiser (or aftermarket mercruiser) parts. When I resealed my outdrive, replaced bellows, gimble bearing, I ordered most of the parts online, it was less than 50% of dealer prices.
Might check out Ebay for the manual...