In JavaScript, how can I get all radio buttons in the page with a given name?

You can use document.getElementsByName(), passing the name of the radio group, then loop over them inspecting the checked attribute, e.g. something like:

function getCheckedValue( groupName ) {
    var radios = document.getElementsByName( groupName );
    for( i = 0; i < radios.length; i++ ) {
        if( radios[i].checked ) {
            return radios[i].value;
        }
    }
    return null;
}