I remember when the M6 was extremely cheap, and readily available. Sadly you don't see them anymore. Apparently the people who were smart enough to buy them (this EXCLUDES me) realize what a practical firearm they have, because when they do find their way on the secondary market they are EXPENSIVE now. Gbroker would be my only guess, but be prepared for Bussekin like secondary market prices.

Remington was importing combo firearms not that long ago, but they were heavy and suffered some reliability issues.

If compactness is the OP's goal I offer this solution....

Marlin Papoose

I've owned one, and regretably sold it. They are, however, still in production and are capable of better accuracy than I've seen with the M6. You do not get the 410 shotgun, but you do get a very lightweight semi-auto take-down 22LR.