So does Everyday Performance. You'll have to contact these carb rebuilders to see if they have a core of the exact model you want. Usually the correct number for a late '60's Bird or GTO will come with a premium price tag. But, there are other numbers that will run just as good, when correctly rebuilt.
I don't know how good the Uremco carbs, that Summit sells, are. Some models are cheaper than many of the highest rated rebuilders. But, one listed for a '68 Bird, has a $180 core charge. and shows a shipping date of 8-29-16, if ordered today. And there is no guarantee that they even have a correct core to build. Not sure what you'd get--IF--or when. ?
The later model Pontiac carbs, such as '77-'78 models are cheaper than most early units. But, they have slightly different linkage, hot air or elec choke, and are 800cfm. In other words, they won't look original, may required slight mods to the throttle linkage, but might be functionally just as good or better.
Summit appears to have one of these in stock. As you can see, these Pontiac carbs have the big "smokestack" vent, which requires an air cleaner base with a hole in it, for the big vent.
Other GM Q-jets, such as a 403 Olds, has the front inlet, but does not have the big vent. These type carbs are shown in the SMI link I posted. Here is a Uremco model.
There are some early models on Ebay. But they ain't cheap.
And, as you can see, the links won't work. But, you can go to Ebay and enter 1969 Pontiac Q-jet, and find some. No idea about the quality of any of these. ???
But, there are other numbers that will run just as good, when correctly rebuilt. 
