Unfortunately the high cotton content rules these pants out for most climates. Cotton soaks up too much water, takes forever to dry, is a sure ticket to hypothermia. Do check military surplus stores, though. I still have a pair of Air Force surplus tropical weight pants, lightweight polyester and wool blend. I bought them in the mid-1980's. The fabric is like cast iron--it refuses to wear out!

If you can find a pair of pants that fit in waist and crotch but have a really long leg length, you could get a tailor to add the zippers to make them convertible.