The term “think tank” seems to have arisen in the 1950s, but there were organizations of that type even back in the 19th century. Many were established to address the problems of war (not successfully I’ve noticed), and others social, environmental and education policy.
The three most-cited conservative think tanks are
the Heritage Foundation, fifth most influential think tank in America.
the Center for Strategic and International Studies, the seventh most influential think tank in America.
the American Enterprise Institute, the eighth most influential think tank in America.
One can only conclude from 5th, 8th and 7th being conservative, that 1-4, 6, 9 and 10 are liberal think tanks and reporters would be quoting them more often. But this list is for my own convenience so when I read quotes, I’ll know whose bias is what.
Other conservative think tanks include the following:
Project for the New American Century (according to Wikipedia, is extinct, 2006)
Center for Security Policy
Foreign Policy Research Institute
Center for Immigration Studies