Hi Aully, I think any fabric store that sells quilting cotton would probably sell invisigrip or a comparable product.
You can also make your own sandpaper dots if you decide to go that route. Buy a small piece of fine grade sandpaper and use double sided tape (like for scrapbooking) to attach it to the ruler. If you decide you don't like it much, Goo Gone or WD40 will remove the adhesive residue.