Not a bad idea about making them. I have to ask the mother-in-law to see if she can do it, as she has a sewing machine.I'd just make them myself. I can buy two yards of felt from Hobby Hobby and make about a dozen or so socks (4" diameter) for the cost of buying one or two commercial socks, depending on the brand. The trick will be findiing someone with a sewing machine that can do the seams (I can't recall what seam is typically used, but I'll see if I can get a pic of mine later today if needed).
If you are set on buying them, I'd order them from bulkreef supply. They sell theirs for like $6 each. Marine depot sells their 4" socks for $5 each.