Been there but never found such sheet myself. Have 3 different ways of overcoming that:
1. Slip the mattress into a single doona cover. 2. Make a single cover by folding over a double sheet and sewing up one end and the side. 3. Clip the underneath of a fitted sheet with a pair of these (both across or one across and one length ways.
I have single beds in the aluminium TeePee and put what I call 'egg cartons' (overlay) on top of the mattress and a single sheet on that, tuck it in and there is no movement. Works well for me.
I get the 'egg carton' from that rubber place and the owner is Clark. I get a queen size one and get them to cut it to size for me. Does both single beds.
Keep Safe on the roads and out there. In the bed as well
__________________
Live Life On Your Terms
DOUGChief One Feather (Losing feathers with age)
TUG.......2014 Holden LT Colorado Twin Cab Ute with Canopy
DEN....... 2014 "Chief" Arrow CV (with some changes)