Anyone could walk into a Chevy dealer and order a COPO -- IF (big IF)they knew about it. Apparently few did as there were only about 1000 units built.
Same would be true about the 68 Yenko. With Yenko's network, you could walk into any Yenko dealer and buy one. Few did as only 64 were built.
I think the coolest 68s out there are COPO9737 L78s that did not get converted. I think I'd rather have one of those with its born with L78 driveline, than have a converted 427. But that's just me....I can't afford either.