I used
Airtex Interiors [
airtexinteriors.com] to redo the interior of my 172. The seats were very easy to redo with the kit they send you.
When I had a 150, I found a very nice complete leather interior, side panel and all, on eBay. It was very easy to install.
Changing or repairing the upholstery fall under owner maintenance. I am not particularly good at DIY projects, but changing the seat covers is not complicated at all. Airtex was very easy to work with and sent me all kinds of pictures on how to change the seat covers. They included all new memory foam and the outside material was well stitched and looked great.