If you look hard enough, you can find a few Republicans Against Prohibition:
The Drug Policy Alliance is running an ad in The New York Sun aimed at convincing Republican delegates that opposition to the war on drugs is a respectable position on the right. The ad quotes Milton Friedman, Bill Buckley, Grover Norquist, George Shultz, Gary Johnson, and Arnold Schwarzenegger (who is by no means a full-fledged antiprohibitionist but has come out in favor of medical access to marijuana).