I must note that the 'wastebasket' felt I found is essentially unchanged after over 40 years. Something similar would be found in the link(s) I posted because it IS a standard commercial product. Some companies will provide various thickness & densities. (Mine was about 1/8" thicker than standard, which lowered the action and took less pressure, but it required shaving it with a razor blade for the thicker bass strings to avoid it touching.
IF one is a serious musician, like Bryan Bowers, and has several instruments, it might pay one to investigate the various options... but if it's merely a hobby and involves only one 'harp, it is probably easier to just deal with the standard dealers. (Note: a dealer won't sell a lot of felt if it lasts 40+ years... it is probably a better business model to stay with the softer white felt that gradually develops grooves and needs to be replaced.) Your choice.