Are Warder's cloaks ter'angreal ? Are they some form of streith? Has this ever been explained?

 

Cos if the secret to making ter'angreal had been lost during the Third Age, where are the Aes Sedai getting the cloaks from?

I could be wrong but I seem to recall that there is a ter'angreal in the Tower that is used to make the cloaks.

Yes, that is correct. According to the WoT Companion, Warders' cloaks are made from fancloth, a synthetic fabric that duplicates its surroundings for a full camouflage effect. This material is produced by a Ter'angreal in the White Tower. 

Ah, thanks!

I don't have the Companion book (I'm only starting book 8 now), so that explains it. ☺️