We needed a palatal expander for our former preemie with eating issues and decided to go with an ALF appliance. You can see how it fits into the mouth here:
https://bronsonfamilydentistry.com/service/quantum-alf-stealth-orthodontics/.
It was pricey, but it worked. We're at the end of the process now and he's in invisaligns—not sure if that's always a part of it or not.
We used Bronson, but I think there are a few others around.