D. Scott Chatfield
First thing, if you're looking for O-scale detail parts, the first place I'd call would be Des Plaines Hobbies. Same for S-scale parts.
The horn is a Nathan M-3.
The bell is a standard "fabricated frame" switcher bell, but looks a little smaller than normal. Maybe because it's partly hidden by the fan shroud. But you might consider an S-scale bell.