Torso height is the main issue. Measure her from bum to shoulder when seated on the floor. With a little luck, she's all leg and will therefore fit in a "standard" CR for quite some time. Height limits are less strict than weight limits on seats: the "final answer" is whether the harness is at or above the child's shoulders and/or the tops of the child's ears are below the top of the car seat shell (which ever comes first). If these are both true, then you're fine. As the child's shoulders move toward being level with the top harness slot and/or her ears are at the top of the shell, it's time for a new seat.
There are techs who have taken an additional class in child passenger safety for kids with special needs. You can search the link below for a tech with special needs training in your area.
https://ssl13.cyzap.net/dzapps/dbza...tegory1=TAB9_CERTSEARCH&Webid=safekidscertSQL
Many (certainly not all) special needs techs work as OT/PT's. In addition, some children's hospitals and rehab clinics have special needs "fitting stations," where you can see special needs seats in real life AND have a trained tech help you choose one that best fits your child, your vehicle, and your needs.
Car seats for kids and teens with special needs are as PP mentioned, vests or belt position boosters with additional support. There are also several that are oversized car seats with 5 pt restraints. Below are links to a couple of manufacturers/distributors of both vests and seats:
http://www.adaptivemall.com/safetycarseats1.html
http://eztether.com/
http://www.columbiamedical.com/special-needs-disabilities-products-categories.asp?SubCategoryID=5
http://www.ezonpro.com/index.shtml
If you haven't already experienced this, you need to know when something is labeled "special needs," it automatically doubles or triples in cost. Because of this,
some health insurance plans
may help pay for a SN seats as "durable medical equipment." (Some durable medical equipment distributors have experience with special needs car seats, but many do not.)
You will need letters from DD physician and therapists indicating the need for a SN seat. This is NOT something that happens overnight. In many cases, the insurance company will deny the request 1-2 times (requiring new documentation and "letters of medical necessity" each time) before ultimately agreeing to pay for the seat. Because of this, it is really important to start the purchase process
well before DD has outgrown whatever seat she is using. You will need the time to select a seat and then get the required letters, documentation and approval from insurance before ordering.