If you find that you don't have a riv-nut and the braze-on just spins, you can drill it out and install a riv-nut. Plenty of people carry them :
http://www.mcmaster.com
http://www.aimfasteners.com
http://www.aircraftspruce.com/search/search.php
http://www.hansonrivet.com/w64.htm
Don't put an aluminium riv-nut on a steel frame and vice-versa.