I know this is an old thread but it just helped me finished a side curtain rebuild. I didn't want to make a special order to England only for felt. The shipping would probably cost more than the product. So I tried the Velcro suggestion, using the fuzzy side. I bought a roll of the black 3/4" wide version with sticky back.
I initially ruined some because once you stick the sticky to any part of the aluminum side curtain it does not want to go down into the channel. And if you let the sticky side stick to any other part of the sticky you cannot get it apart. So, more ruined material.
I found the solution by putting Talcum power on the sticky side so it is no longer sticky. Then you can get it into the channel evenly. I also found that the Velcro does not like to bend around the corner curve. It was there that I used about 5" of the old felt.
Hopefully, someone searching this thread will find it helpful as I did.