Fixittools has the Malco NHP-1 Nail Hole Slot Punch right now for $25.42.
The NHP-1 is a handy tool for siding installation. If the hole in the siding doesn’t line up with a stud or nailer, you can use the punch to turn the hole into a slot or add a new slot, allowing the nail to be positioned correctly. Seems like a good way to solve an annoying problem, and one that if not addressed could cause a poor installation of an important element of your house. The NHP-1 is also available from Amazon directly for $29.90, but with free super saver shipping.