The local AutoZone in Denver, CO had a three pack of allen hex sockets (12mm, 14mm, 16mm) for something under $10, as I just recently ran into the same problem of trying to find an allen wrench or socket to remove the oil screen (turns out it's a 14mm). Snap On was also suggested by the Duc dealer, but I of course had the oil drained and the bike sitting, so visited about 8 different auto, hardware, and tool stores before finally finding what I needed at AutoZone.
Better luck to you!