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I have an old 5" Porter Cable ROS that uses the 5 hole pattern PSA disks that aren't that popular now. I wanted to order sanding disks appropriate for finishing but each grit comes in 100 disk rolls and are roughly $27 per roll plus shipping. So I don't want to order a particular grit unless I really need it. Also, I'm uncertain if I want to invest this much money in sandpaper that matches the hole pattern for an old random orbit sander (what if I need to buy a new one soon and it has a different hole pattern?).

I happen to have the same paper, Carborundum Premier Red Dry Lube, in P400, P600, P800, P1000, P1200, and P1500 grits. Any reason I couldn't use an old disk as a template and cut disks, including the 5 hole pattern, from the sheet paper stock? I could then spray the back of the disk with 3M Super 77 spray adhesive right before attaching to the ROS so they stick like a PSA disk.

Maybe then I'll figure out which grits I need to order so I don't have rolls sitting around unused. Think it would work? I've heard you can buy a punch to cut the hole pattern that matches your ROS but I've never seen one. If true, I could buy paper without holes and if I change ROS, I would just get a different punch for the new sander and could still use my existing sanding disks.

yes you can
If you have a drill press and vise you can make a simple punch.
Drill a hole of the same size , You can make it a bit larger or smaller. If you have say a 3/8 drill take an old 3/8 bit and grind the chuck side flat and then chuck it upside down then use the press as a punch

That's not a bad idea.

For the record, I tried this the other night and it worked. Used P400 and used P800 disks cut from 9X11 sheet and sprayed with Super 77 3M glue.

Way to go Darryl. I've made discs for my combo 6" disc/belt sander this way for years.....

Kevin