Any paddle with an M14 thread will work.
Make sure you get the proper paddle for the proper material.
I have recently purchased the MX1602 and have looked around to find a proper mixing paddle. Unfortunately I could not find any that were much cheaper than the Festool offerings so I got one from Festool for €50.
For mortar the Festool HS 3 R or off-brand equivalent should do. Festool offers the HS 3 in an R and an L version, which indicates if it moves material from bottom to top or the other way around.
