The old GW Empire freebooter sprue had one attached to an arm.
IIRC proper name for the kit was "Empire Militia" though the same sprues were also released as "Mordheim Mercenaries". In any case that is a pretty decent looking blunderbuss, and it is possible to shave off the hands holding it without damaging the weapon.
As for other sources, I checked TAG and they did not seem to have any, though they have a large selection of firearms. Though they have trumpets that you could cut off and glue to the ends of musket barrels to make them flare out.
I also checked Alternative Armies - they have an interesting selection of Flintloque guns, but nothing very blunderbussy. Though if you need carrots or sausages, that would be a place to check.