The Eastern and South Eastern parts of the USA have many mosquitos especially near the sea coast and near rivers and lakes. Florida has many mosquitos. The Northern states and the mountains also have mosquitos in the Summer months. California, and most of the Southwest desert areas have no mosquitos except in the high mountain areas.
You will find almost no mosquitos in Las Vegas, Show Low, Phoenix, Tucson, Alamogordo. You might see a few mosquitos in Flagstaff, Santa Fe, Bryce Canyon or Colorado Springs because they are in the mountains. In Wyoming, Montana and South Dakota you can find some mosquitos because that is in the North.
You can find mosquito repellant lotions and sprays in every outdoor or camping store, and also in many drug stores.