Where do you come across stuff like this, if you don't mind me asking? I'm on a Micro Machine and Action Fleet FB group now, and people on there are showing off box art and stuff... but I've got no idea where they're coming up with it all. I tried some googling to no avail... I'd love to get my hands on something like the designer's notes. That's amazing.
I'm the Admin of that group.
My concept art came from another collector, who obtained it directly from the artist. I just picked that up a few months ago. As with most collecting, it involves being in the right place at the right time. Networking with other collectors, working out deals, that kind of thing. There's no one source for anything, you just have to be persistent and hope for luck. The box art posted by the other member was obtained from an auction house over the summer. It kinda flew under the radar. I was not lucky that day and didn't see it until it had already closed.

The Action Fleet Death Star playset McQuarrie sketch Mike posted to the group came from a Rancho Obi-Wan charity auction a couple years ago. He was lucky enough to win it for a very good price.
I'm primarily a vintage collector, but Galoob stuff has always been one of my passions, since it's very nostalgic for me (and it's awesome, obviously). But back in the day, when Hasbro bought out Galoob, the vintage prototype collectors had a lot of Kenner contacts. So their interest in Kenner meant that they would occasionally find Hasbro pre-production stuff, and some of that Hasbro stuff was occasionally Micro Machines. They weren't overly interested in it, so a lot of it came to me, since I was the vintage collector they knew who collected Galoob prototype stuff. That's where all of the unproduced Micro Machines Gearmates boxes came from, for example. Vintage collectors fished them out of the trash at the old Kenner building and they eventually found their way to me.
For other 3D prototypes, the head of Galoob's model shop used to sell on eBay in the late 90s and then again around 2007-2008. He kept a lot of stuff and since it rarely sold for much, he and I worked out a deal for everything Star Wars he had left. By that point it was pretty picked over, but I wasn't complaining.
Tommy