I was in the same boat a few months ago looking for transportation from the airport for our family. I need a booster and a car seat that can fit a five year old who is over 40 lbs.
I emailed a bunch of the companies usually recommend on here, and didn't get any reassuring answers. Either no response, can't commit to having a car seat for a 40+ lb child, or they only have car seats that are suitable up to 40 lbs.
I finally found (and booked) Cast and Crew Transportation
https://castandcrewtransportation.com/ (they also have a Facebook page and group). They have car seats and boosters available (including high back boosters) and post the specific models they use on their website.
We aren't traveling until next week, so I can't comment on the service yet, but they have been responsive so far.