If you can't find a wider hammer, you might want to consider a hammer extension.
When you put a scope on a lever action rifle, there isn't enough room to get your thumb on top of the hammer. The extension slips on the hammer and is held in place with a little set screw. It sticks out to the side of the hammer so it can be cocked or lowered easily. I know UNCLE MIKE'S, among others makes them. Take a look, it might work for your application.