Since overseas shopping can be too expensive and complicated in our country, some local repairmen are using alternative methods.
They are using glue for catching mice or rats, that is adequately thinned for this purpose.
They say that the basic components in the mice glue and the "real" cloth sealer are the same.
I've never tried it, but this method is maybe worth investigating, as the alternative for the rest of us who do not have access to the fancier stuff.
Here is the example of vintage Tannoy K3809 driver restoration, you'll have to scroll down to the mice glue cloth surround treatment part:
https://www.zenmod.in.rs/another-tannoy-k3809-adventure-read-revitalization-again-3-previous-episodes-cumulative-with-finish/
Another link:
https://www.zenmod.in.rs/tannoy-k3809-hardened-cloth-surround-issue-solved/
I know that this type of glue is forbidden in some countries, because it is a very cruel method for killing mice and rats, but this way at least it can be put to a better use.
Here is the example of the glue they are using: