My daughter has been in an electric wheelchair since she was little. Jay was the first company that made the gel cushions. You can buy them "off the shelf" or customize the inside by placing the gel packs where needed. There may be other manufacturers out there now, but Jay is still the one we ask for every time my daughter gets a new chair.
When you go to a medical supply store, ask for someone who is trained to customize clients' wheelchairs, as they can recommend the size and configuration for your condition. We have followed Tony around to his many places of employment because he is so good at customizing equipment to fit our daughter's unique needs.