There's the Second Amendment Foundation (SAF) and Gun Owners of America (GOA).
I also recommend taking a look at Project Appleseed, a non-partisan group of men and women (known as the Revolutionary War Veterans Association) who are committed to upholding the values and principles of America’s founding fathers. They use rifle marksmanship instruction as a gateway to help bring our nation’s history to life and to show that many of the values that our forefathers relied on to win our independence are still very much in demand today.