The only ammo that I've ever seen was sold as a collectors item for $3/shell, with a 3 shell limit.
That is hideously high!
Fiocchi still makes it:
http://www.fiocchiusa.com/cat_rimfire.html
http://www.outdoorsuperstore.com/product.asp?prod=294829&294829
"...try a round a skeet just for the hell of it..." That's all it'd be. Max range is about 15 yards. 1/4 oz shot charge at about 600fps. 87 ft/lbs of muzzle energy.
But, can some one answer my questions on the BB caps?