Some places where you can buy ANSI B16.5 flanges online include Coastal Flange and Texas Flange. Any hardware store that sells flanges should have ANSI B16.5 standard pieces in stock, as ANSI B16.5 flanges conform to equivalent international and American engineering standards from leading organizations.
The American Society of Mechanical Engineers, despite the name, is an international body that regulates the dimensions of tools and mechanical parts, including flanges, in over 100 different countries. Its standards are voluntary, but many manufacturers follow them to ensure that their parts are compatible with most mechanical parts from around the globe.
While ASME is the leading global organization for engineering standards, the American National Standards Institute is the organization that sets engineering standards in the United States. The ANSI B16.5 and ASME B16.5 standard both cover flanges and are equivalent to one another, so flanges that conform to either standard are often labeled ANSI/ASME B16.5 and can be used interchangeably.