We bought some flat sheets, measured and sewed to fit.
Works great except we should have started with king sheets. We used a queen and it slips a little in the middle with the narrow foam mattress, not enough material underneath,
Cheers,
JFS III
Depending on the RTT size this is what we did and may work for you.
We got twin extra long (for college dorm sized beds) stretchy jersey sheets. Because of the extra length and the stretch they fit our 47" RTT perfectly.