[vash]

I see contractors do some tight radiuses (radiaii?) by using plain old plywood. they cut a bunch of shallow cut on one side so the panel can bend. they line with a flexible sheeting if they need is super smooth.

line up the circle. hammer in a bunch of forming stakes and then work the ribbed plywood around the circumference. And line with a form liner.

I do however think you can BUY a round form.