Don't remember seeing on on the web, but you can call the place that you brought it from and see if they sell it. I know that MarineDepot among others have generic bubble counter if you don't mind about not a Knop.... I would just DIY one from acrylic tubing, etc.... HTH.
I don't think it's a regularly stocked item anywhere but places who sell Knop products like Marine Depot will likely be able to get them (if you can wait a few weeks). I'd just go with a generic bubble coutner like dupaboy says; they perform just as well if not better.
