I have heard that there is a ruling of Sayyid Sistani stating that it is Makruh to stitch a Kafan. Is this the case and, if so, where can I find this ruling?
Bismihi ta'ala
You will be able to find the view here.
https://www.sistani.org/arabic/book/22/1728/
The emphasis is that the kafan be of one piece of cloth, and if needed to be sewn, then its own strings should be used. Furthermore, A.U. Seyid Sistani does not completely agree that it is makruh. Please see link.
And Allah knows best.