Click the RESOURCES button on the menu bar and scroll down to find the Parts entry. It lists many salvage yards and parts outlets.
You can find generic size/shape arm rest covers on Amazon and Ebay, and many places that has automotive upholstery items. You will have to use your own judgment as to whether they fit and look OK. You could also get them recovered at most any upholstery shop, and maybe even get OEM replacements if you know the furniture brand. Most coaches have seating from Flexsteel or Villa