There are a few online sites that you can get parts from. you can do a google search for Johnson or Evinrude Parts (some places are cheaper than others on some stuff.)
Don't forget https://www.iboats.com/Marine_Store/dm/
I have bought a water pump rebuild kit from ishopmarine, as many places don't list it for a 4 deluxe/4.5 Evinrude, and while I did get the part alright, I have heard bad things about them from numerous other people. I really see no reason to get OEM parts if you can get the part you need from Sierra or Mallory. Both are perfectly acceptable.
Also depending on what you are looking for Ebay is sometimes cheaper. Just takes awhile to find what you are looking for. I have a '76 25hp johnson that I just got the whole transom assembly for under $25 shipped to my door.