I too had the rubber boots deteriorate.
In my searches the closest match I found was Mirrors made for the King Long Bus. I never found an exact boot replacement but did find whole mirror assemblies out of China through Alibaba
https://www.alibaba.com/product-deta...7121.77.suikEi
model B-11013 has the correct mounting and comes as close to OEM as I could find a set costs $100 but the shipping is the kicker at $300