There is one that circulates on eBay occasionally. However, it is a restamp. The price is usually around $300. It is a poor restamp, too.
Be sure to check inside the bowl of any you buy. Years ago, these went through the octane booster period where many ran it trying to make up for the bad gas in the late '70s. The octane booster didn't help the performance much if any, but it did play havoc on the pot metal of the fuel bowls.
The 68 carbs are extra expensive and rare because the corvette numbers were only used on corvettes. In 1969 the corvette number carbs were also used on many other chevys, so there are more of those carbs around.... but even the 69 carbs are expensive.
Problem is, even if you found a nice 7028209 and restored it, you probably won't have the right date anyway.
I have an Edelbrock Qjet. One of these days I'm going to stamp "7028209" on it! And the right date too.
It is the unobtainable one. Unless you are judging the car, buy a Q-Jet for any '68, set it up to the same jets and rods, and use it.
If you find and buy the 7028209, keep it dry, clean, and on a shelf except for a judging meet.
Yeah, tell me about it! I've seen "cores" for $300.00 - 400.00 here and there. Right now, I have a incorrect year Q-jet that Lars rebuilt for me that works great. I just wanted a correct year Q-jet for my '68.