These are increasingly difficult to find. When I still had my Benelli 350RS it also needed a new filter, which was the same as the Honda CB350/400/500/550 Fours. But remarkably these looked very similar to the PT filter, with the exception they are not flat on one side. So the easy thing to do was to make a spacer ring to accommodate for the flat part of the original filter. And it works.
While dismantling few GTV engines I noticed the oil filters were identical, and these may well be the next best alternative to the Champion X315 oil filter: