Where do custom exceptions belong?

I’m curious where custom exceptions are usually stored in a rails app.
I want to create a UserException that I intend to throw if the user
tries to do something that causes an error such as trying to delete
something twice or access a page that they don’t have permission to
view. I’ll probably eventually end up with several sub classes of
UserException. Let me know if you have any recommendations on where to
store them.