There are lots of post about 'cleaning ' armrests. Finding replacements can be hard if impossible especially for older years. I have seen simple but fairly ugly fixes like putting socks over the rest. I thing the biggest bang for the buck assuming the fabric is not torn, is painting with a fabric spray paint.
If the issue is mold and not so much yellowing, there are posts about soaking the rest in various solutions. I also remember folks mentioning that removing the rests to soak them can be a tricky job.