Duck Pin bowling is found mostly in Maryland. Interest in the sport has been on the decline as many duck pin centers have closed and lots of it's followers have switched to ten pin bowling.
Coming across a full set of duck pins can be difficult especially since few are made each year and they're very expensive. Getting a duck pin ball would be easier but without the pins it's not that great.
You might be better off ordering one of those In-School Bowling Kits that the YABA/BPAA sell to schools. They're designed to teach children ten pin bowling. They are also on the steep side, I believe they cost around $100-200. If you're interested in that e-mail
yababowling@bowlinginc.com , and ask how you can order a "In School Bowling Kit."
The cheapest solutions would be something like the following from
Amazon.com:
Stats Big Ten Bowling Set Big Strike Bowling Set Silly 6 Pins Bowling Game