.I've been busy offline...surprised no one else has jumped in.
You should be able to find a fitting that'll work as your pumpout fitting on the tank at any good hardware store that has a plumbing department...you might want to take someone with you who's had a little more experience with figuring out what'll work.
The rest you'll need to find at boat stores...a deck pumpout fitting, a y-valve, some hose (measure first so you'll know how much to buy)....a thru-hull fitting (see attached photo) for the vent line...a y-valve (aka diverter valve) and a manual diaphragm pump. Again, talk to other boat owners who've had more experience that you've had (don't feel slighted...none of us were born knowing anything about boats, we've all had to learn everything).
Spend some time online looking for all these things to learn brand names and even what they look like, etc. Then come back here to ask for opinions about which ones people are happiest with...and to ask the additional questions you're guaranteed to have.