The Childrite Therapy Chair, designed for children with special needs, is now in production.
 
The Childrite seat encourages children to strengthen their trunk muscles while stabilizing their hips.
 
According to www.childrite.com, "It's Bumbo-style wrap around back provides support to help your child sit up on his or her own. The extra wide leg openings and broad hip area make it comfortable and useful for a long time. Great care has been engineered into the seat to insure that it is soft and comfortable while remaining sturdy and supportive."

One note... this is a chair for home use and isn't designed for the car or bathtub. 
The cost of the Childrite seat is $199.95 but the website (www.childrite.com) has a special launch coupon available. 
 
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The Bumbo Seat is another option and it sells online at Rehabmart for around $79 http://www.rehabmart.com/product/11991.htm