Can't help with your query, Nessmuck, but any store who sells Denim in ounce weights, sells exceptional patching material in my opinion.
I like the 10, 12 and 14ounce versions. I use the 14 ounce, for the small 15 bore (.675") balls in my .14 bore (.690")rifle. I measure the
14 ounce at .034".
I also found some .022" light canvas that shoots just as well, but is slightly harder loading than the 10 ounce denim which also measures
in that realm. I use both of those materials with balls from .005" to .008" under bore size.