I have no idea where you'd find a commercial pattern, but I have an idea about how you could make your own!
Buy yourself a big beach ball, and slice about 1/3 of one of its seams open (to give yourself a fake 'waist band'. Cut a hole (slightly off-center) on the left side of the uncut bottom hemisphere, and another on the right side for your other leg. Once you figure out how much space you need for your legs, you'll basically have a ball-like shell that you cut into pieces for your pattern.
You'll definitely need to look into some supporting undergarments to keep the ball shape, but unfortunately I don't have any particular ideas about how to tackle that bit at the moment.
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