Re: 40hp Johnson lower housing unit??
Get them from an Evinrude/Johnson dealer. Some general marinas have them, too. No substitutes. If yours has washers, use washers. If it has o-rings, use o-rings.
The o-rings are easier to find, so you might want to get a set of plugs that use o-rings when you're in the dealership.
These washers and o-rings are responsible for many of the leaks in lower units, and should be replaced each time you remove the plugs.