There are zillions of them available. It really depends on what type and size you want. Hope this helps ... http://lmgtfy.com/?q=cushion+for+mobility+scooter
Little experience of mobility scooters but "juddering" sounds as if it needs more than a cushion. Either a cure for what is causing it, or at least better suspension. I'd suspect a foam memory cushion might be a good idea though.
OG ... I'm afaraid scooters often have no suspension for safety reasons. If suspension is compressed, then tilt may occur, along with small wheels and a sloping pavement ... A recipe for disaster.
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