On the package it says http://www.lymanproducts.com so I imagine they do sell them through the mail but I allways just picked mine up at the gunshop. The part # for the flints is 6030106 if that helps.
I have been using TC premium agates for about 10 years in mine. Easy to find at Wal mart, Dicks and Gander. Never had an issue. The regular TC flints , although cheaper,do not last more that a few strikes for me.