You have any piano tuners/repairers/restorers near you? They may have some felts or give you some sources.
When I was active, a lot of the Korg keyboards were made by Yamaha. No secret, the circuit boards for the contacts still said Yamaha right on them. I used to stock DX7 heys for my Korg customers. I used to buy my Korg keybed parts from Yamaha as they were cheaper. I did have a selection of felts, but do not know if I still have any. I know I found felts somewhere, because I used to replace them in Kurzweil keybeds. The counterweights would clatter when teh felts wore out.
I know you want to match what was there. But if memory serves, is it OK to have the whole length consistent, even if a little more or less stiff than original?