1) On eBay, or a similar auction site.
2) Many Bowling centers will sell you a new bowling pin. Expect to pay around $20-$25.
3) Some leagues, known colloquially as Hav-a-Ball leagues, can provide you with a themed bowling pin (usually in lieu of a similarly-themed bowling ball) as part of the cost of participation.
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You can find Duckpin Bowling Balls and other equipment online, such as sites as http://www.bowlerstore.com/c-86-duckpin-bowling.aspx or http://www.thefind.com/family/info-duck-pin-bowling-balls.
Bowling pins can be purchased through sports goods stores or other such online websites. It's possible that bowling pins may even be purchasable via a bowling alley, if they happen to have any pins that they are getting rid of.
A good source would be eBay; duckpins (both used and new) are offered up for sale there on a regular basis.