Copied from the below link:
Various transparent perforation gauges on the market offer a range from 10 up to 18 (Instanta) or 8 to 16 (Linn’s Multi-Gauge). Unfortunately, the modern-day self-adhesive stamps have gauges ranging from 6 to 9, which fall below the Instanta gauge. The gauge I came up with, ranges from 5.3 up to 10.
Info on how to aquire gauge:
http://www.adminware.ca/esg/perfgauge.htm
Puzzler Note: It is not on stiff plastic but on flexible thinner plastic. It is nice and works (I have one). I also have a Unitrade one mentioned by David, it goes from 10 - 18. Nice but for the Canada Flower and Olympic coils it doesn't work. I am not sure if the US stamps are low perfs like the Canada ones have.
Doug