A lot of kids who have medical issues that require them to be in a car seat long past the usual age can sometimes get a prescription and have some or all of the cost of the seat covered by insurance. Always check and see if this is an option for you!
This is a new seat that harnesses to 115 lbs and has 22" top slots (important for tall kiddos). It's called the Roosevelt by Merritt Manufacturing. It runs about $1000.
Here is a site with tons of special needs seats to choose from:
And don't forget: Kids are safest rear-facing for as long as possible, and this is especially true for children with low muscle tone. Most kids can easily rear-face to about age 3 in car seats like the Britax Marathon and the less pricey Evenflo Triumph Advance. If your special needs child is small enough to be rear-facing, they really don't need a special needs seat yet, as these all only face forward and are usually very large.
Here is an album of kids "extended rear-facing". They are much safer this way and contrary to popular belief, they are not usually uncomfortable with their legs folded.