I first look at the winnebago parts page to see if I can find the part and number.
https://catalog3d.winnebagoind.com/menu/Parts.htm
From there I go to good old Google and see what I can find. Most times the numbers do not match so I call Lichtsinn. Those folks are really great, every time I have called they have been there for me. I have ordered parts from them and I have asked them for more part number information so I could do my own looking.
Once I found the correct part number Amazon was my place to go. I had to replace the passenger side mirror this past summer. I saved over 50% by going with Amazon over Velco (mfg) or Lichtsinn.
good luck