To follow some of the comments above, most of the aircraft interiors I changed were recovered by a local upholstery shop. Brilliant craftsman who could recover anything. As mentioned, seek out local aviation mechanics, but also visit the local FBO (aircraft gas station) at the airport on a Saturday morning for a cup of coffee and talk to some local pilots about this idea. Not all recovering requires a mechanic to be involved. Maybe the pilot who suggested this to you can help. And while being close to the airport is a plus, the shop I used was a 20 minute drive away.
Yes, there are various rules and regulations on the aviation side when it comes to disassembly/reassembly, materials used, etc. but there are none pertaining directly to the cutting/forming/sewing of the upholstery materials. However, the finished product must meet a certain standard that the mechanic or pilot/owner will accept. I was never dissatisfied with any finished product received from my local shop.
Also, if you have the equipment, carpeting and headliners can be included in the services offered. Nothing like a complete recover top to bottom. Removable padded seat covers were also a big hit.
As a side note, most aircraft must be inspected once every 12 months, which usually entails the aircraft to be disassembled/reassembled over a period of time. This is when a lot of pilot/owners usually comply with upholstery upgrades and the like. The inspection requires an additional authorization for the mechanic, so it is something to inquire about with the mechanics you talk to.
Just keep it simple and stick to what you know best, upholstery, and leave all the other details to the mechanic or pilot/owner. Unfortunately, my local upholsterer passed away a number of years ago and I quit o the service. However, if he were still around, it would have made a great retirement job to earn some beer money. Post more questions if you need.