We've used Tiffany Town Car each time and they have been able to provide a car seat for my toddler each time. You do have to note that you need one when you make your reservation.
I used 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 used to use Tiffany Towncar and they were excellent for years until this past year. Don’t take my word for it, look at recent reviews if they are still up. I used MCO2Disney this past trip, which I saw on pixie dusted mama channel, and they were excellent. Up to 3 car seats free of charge instead of nickel and diming you like some.
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