Allie in our local club we have 4 different 939 series trucks but every one I know of has just the drop side beds on them including mine. With that said I will certainly put the work out and see if anybody else knows where one of these can be had, If I hear of something I will sure let you know the contact info! Good luck in the meantime!
A deuce bed will work or you can cut down a 14ft bed from a 5 ton. Trying to find a drop side deuce bed is like looking for a pot of gold. There is a LMTV bed for sale in upstate NY, which would work as well.