What material is used to make American uniforms in world war 2?

Generally cotton for summer weight items and wool for winter weight.

In WWI the US had experienced a wool shortage in attempting to outfit its army with winter dress. This led to a government subsidy of the Alpaca goat farming industry. Within a decade or two after WWII the military had abandoned wool, which is smelly and heavy when damp, in favor of much better synthetics. However the subsidy to the Alpaca farmers continued for another fifty years. Just another example of government efficiency at its very finest, and further evidence of why we all ought to be happy to send all our money to our incredibly wise leaders.