Buffered vitamin C is a form of vitamin C that is neutral PH, which means that you can take extremely large doses of it and your body will tolerate it. Vitamin C, ascorbic acid, you can take very little each day.
I don't have a great source, I buy "NOW" sodium ascorbate which you can find online easily (amazon.com even). I think almost any source of sodium ascorbate is fine, but then again I don't plan on using it every day.
Maybe someone else can chime in with a good source.