Do you mean that sponge sock that goes over the filter to keep out dust etc? Just wrap some cheesecloth around it and save a few bux is what I'd do. Not too thick or you will reduce air flow. I don't even see a need for that if you have clean air in your grow room. I have anti-static cloth wipes on all my incoming air vents so even stuff that's been sitting on the shelves for a year is dust free. I tried HEPA filters but they were way too restrictive.
I picked up an off brand charcoal filter. I wanted to wash up the pre-filter- so it kinda ran my thinking into buying a spare pre-filter. Getting back with the original company- not possible. Manufacturers in both us and Canada using different lengths. I wound up buying one 2" too long and I cut the end of it off.