I don't recall ever seeing a tie.
CMOT Dibber used to do an Unseen University scarf, but looking at his website,
it doesn't look like he's doing it now.
There's lots of Discworld stuff for sale at PJSM Prints.
But again, I don't see a tie there.
Sorry I can't be more helpful - welcome to the site and good luck!
Edit - I forgot The Discworld Emporium
- lots of good stuff there - no ties though.