King plain sheets fit - they are just long and you tuck more in. Fitted sheets & pads are the pain to find and very expensive when you do find them. When we had one that size, Nancy finally modified some standard fitted sheets herself. Next time we bought a rig, we made sure it has a full 84 inch bed.
Will any of these work? http://www.camec.com.au/50-Soft%20Furnishings.htm
And here's a tip that uses suspendors to tightly tie in oversized covers: http://www.fmca.com/fmc2003/sepmag/tips.asp#Tight%20Bottom%20Sheet
Some Rv stores sell mattress clips, which are sort of like an old-fashioned garter, which pull the corners together to make oversized sheets fit.