If your model is fairly new or the past ten years or so, go to a local Mercury dealer and get a service manual specific for your motor. Parts can be ordered through them as well.
If you live a ways for a local dealer or marina, then you can try a online parts store.
Here is online parts store that is reliable and decent prices.
https://www.marineengine.com/
For Mercury they only have aftermarket manual like Seloc. If you can try to find a specific year model service manual for your motor by Mercury. Sorry, I wish I could help you with locating a Mercury manual but I'm not sure where to point you other than a dealer.
Maybe a member can lend a hand in that department.