For original linkage I'd try Tony LILRED66, he specializes in 4-5-6 Chevys and may have what you need:
http://67-72chevytrucks.com/vboard/member.php?u=17302
The other options are ebay, wrecking yard, craigslist, etc.
If you want to return to an original pedal, I'd look for an NOS item on ebay. I bought one of the aftermarket pedals, and within a few months it was bent into a C and started sticking on the ball end of the linkage due to the bad angle. Cheap offshore steel inside, very disappointing.
Since I had already tossed out my original pedal and preferred the early gasser look, I upgraded to a 60's era Cal Custom aluminum pedal with a block of delrin glued to the back to act as a bearing surface against the original ball end of the firewall linkage.