i've seen multiple people mention they found this "sock" made of stretchy material that slides over a sonosub quite well. none of these people mentioned where they got it..
Any fabric store will have what you need. Just look for a roll of strechy material that is a tube in the color you want. I used a thick T-shirt material and doubled it up.
My honey made one for me on her sewing machine. She say's to just measure cut and sew. Don't forget your seam allowance.
If you don't have a honey with a sewing machine of your own. Or your affraid of a sewing machine (like me) you can look up in the yellow pages clothing alterations or tailors.
I have found various types. Some are course like those white sports socks with the ribs running the length of the sock, and some are more fine almost like a thick pantyhose. I like the more fine variety. It is more like the material used on speaker grill covers. I got mine at a fabric store called BouClaire. I am not sure if Fabricland has the exact same thing (they have the more course stuff).
Black sock like material called ribbing should be available at any fabric store. No sewing required. Cut and pull. Allow a foot or two extra depending on sonotube heighth. Length shrinks when stretched tight over the tube.