Run up mine out of canvas used for making ironing board covers, as no linen was hanging about spare. Made a new roller cover for my Pfaff ironer as well. Also made some ironing board covers out of Nomex fabric (picked up a nice amount on eBay for pennies, military surplus).
Aparently many makers of ironers didn't bother producing roller covers as it was expected the housewife would run up her own. Speed Queen and Ironrite are the only brands one runs across, besides the modern Pfaff and Miele covers. As the seller in the auction copy states, roller covers for ironers are pretty easy to make. Really only need a hem on each end and drawstrings. Many also used your method of just rolling clean muslin,canvas or linen over the padding as a cover. Long as it was nice and tight, that is all that matters.
Wonder why the seller would think all old Ironrite covers would be bad old stock? If the items were stored well, and were not used, they should be in decent condition. Purchased some vintage Speed Queen roller covers and padding from eBay and both were in good shape.
Tried a cover made of linen on my Pfaff once, and didn't really like the results. Found heavy canvas/duck better suited to the task. Ironrite.com sells custom made covers for Ironrites in linen and cotton as well.
L.