Morporkian students can learn it at the Guild of Assassins' Language and Music Classroom. This will give a small amount of spoken and written. It is unknown if doing so prevents you from using other language schools and training facilities, as it does when the Kompte teaches every other language.
Or there's always the adventurous route:
You can learn Uberwaldean by doing the "Intoxicating Linguist" quest.
Good places to TM it include:
- The square in Escrow (during the daytime).
- Mouldium Carpetsmith and fairground game workers verbally list each of their goods.
- Shopkeepers in Escrow tend to be chatty when entering and upon listing.
Throughout Escrow are helpful street signs and in the center of the square are notes listing carriage stops. Most shops have a sign outside to read and a fair few have ones inside. The town hall also contains several rooms with readable items.