Various sources. Made by Goodmark and sold through dealers. These repro full quarters are ALWAYS on evilbay. full quarter goes for 299.00 to 375.00 plus or minus, depending on who you buy from. Prices don't include shipping. Punch in key words "firebird quarter" and you'll find them. I think some body shop supply houses are authorized to buy so you may check locally to have them ordered for you.
In case you didn't know, the 67 differs from the 68 as the 68 version has the cutouts for the arrow head side marker lights. Full quarters would defineately use less bondo but cost 3 times the price of the skin. I did skins back in the early 80s on a 68 when full quarters were not available for a bird yet. To top it off, I had to buy skins for a crapmaro and section in the louvered area from my existing quarters. I will probably hurt my arm here patting myself on the back but it came out looking damned good, and it was my first attempt at major body work too. Good luck whichever way you go.
Give the laziest man the hardest job and he'll find the easiest way to do it.