YES! I've seen one! HA!
The one I saw was for a milled receiver. The guy I bought my pick-up truck (about as ugly as a certain S10) from built one for his MAK-90. He lived out in the sticks and didn't care what the law was about US parts. Oh well. Anyway, he just made a solid "buffer tube" with an oval on the end to fit in the milled receiver. He just made it out of a plastic block, used a lathe to round it off, then grinder, dremel and file to get the shape right. "cut to shape, file to fit, paint to match". You could probably just take a real AR buffer tube for a CAR and the front half of a stamped plastic stock and epoxy the two together. With the empty tube there would be all sorts of possibilities for storage, and reinforcing the joint.
So I don't know who sells them, but if this guy could make one, it can't be rocket science. I'm sure there's somebody doing it on a commercial scale.
Ross