There’s been talk of using leather covers made for the Honda Odyssey that are available on Amazon and EBay but I don’t believe I’ve seen a picture posted on here of one in action. There are also aftermarket replacement arm rests available on EBay and I have seen a write up and pictures of that. Try doing a google search for T1N armrest
I’m putting together a write up, but I got the leather Honda Odyssey covers to work. The first pictures are the cover simply slid over the armrest, which could probably be steam shrunk to fit better but I didn’t like it. The last pictures are what happens with a bit of stitching
I purchased a sheet of vinyl and used contact cement and a heat gun to form it over the existing arm rest. Best to do this while you still have a full existing arm rest. Once you're missing foam, you need a new one.