If you're looking for a car seat like a child or toddler would ride in for safety and you're needing an adult sized version of a toddler car seat you can look into special needs car seats.
They do make the larger car seats to help safely restrain older kids and adults who need the safety restraint although they are more expensive than a toddler car seat.
Search google search for special needs car seats restraints and there will be several websites that show up that sell the adult sized car seats.
These car seats are like toddler forward facing car seats that have the harness but they are made for special needs adults.
They are around $1,000.00 though but if you need one or want one and have the money they do have them.