21st Century Buy American Act This bill establishes additional criteria that must be met before federal agencies may grant exceptions or waivers to Buy American requirements. The bill also (1) requires the General Services Administration to establish a public website with details regarding waivers and exceptions that have been requested, are under consideration, or have been granted by agencies; (2) increases the domestic content percentage requirement to 60%; and (3) authorizes the Department of Defense to establish a program to make or guarantee loans to certain domestic manufacturers.