Where do people order legal polycarbonate?
www.robosource.net is the most convenient place with good prices. You can also get it from McMaster, MSC, and other industrial supplieers. Or you could steal the plastic from the turning point flags and platforms 
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If you use the flags on a comp robot you will need to do something to remove or mask the color since they were specifically design for image sensor detection.
+1 for robosource
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Good point. Platform plastic is probably the best choice. Just have to watch the size (12x24 max).
I think that the platform plastic (not sure about the flags) is actually slightly thicker than what is legal for VRC. So unfortunately that wouldn’t be an option for robot plastic.
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Depending on how the flags n stuff are all colored, you could use a planer or angle grinder to remove the colors on them. It would also help reduce the platform widths to a legal size.