G100L is approved for all piston aircraft [1]. The only reason someone couldn't buy from them would be obstinacy.
[1] https://www.aopa.org/news-and-media/all-news/2022/september/...
You still have to go here (https://stc.g100ul.com/aircraft/) and buy paperwork that makes your individual airframe and engine legal. What the FAA approved was for GAMI to sell that paperwork for all piston aircraft.
[I am seriously curious; on the one hand, owning and flying a small plane is an expensive, privileged hobby which tends towards the lower-upper-class demographic, but it also requires complying with a shit-ton of government regulation and direction already.]