[Yamaho1]

You can find them on many of the online GM parts stores.
They are quite pricey for what you get. The lowers are different for gas or Diesel. Verify the part numbers are correct for your mirrors.

Chrome Tow Mirror Upper caps
23444117
23444118

Chrome Tow Mirror Lower caps - Diesel ONLY for the sensor
23444121
23444122

http://www.gmpartsdepot.ca/a/Chevrolet_ ... 15633.html

Chrome Crew Cab Window Moulding
22798969
22798970
22798971
22798972

 

[Yamaho1]

Noticed afterwards you do have gas, so your chrome lowers would be a different part number I believe.