Well, they show up on Ebay from time to time.

But if you want a specific ball, I think you need to call the PBA in Seattle.  One of the kids I coach on Saturday mornings wanted the Odor eaters ball.  So his mom called Brunswick, they told her to call somewhere else, and eventually she ended up calling the PBA headquarters.  I believe she paid $116 plus shipping.
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The clock on the wall says 3 O'Clock...last call...for Alcohol!
"I love a martini -- but two at the most. Three I’m under the table; Four, I’m under the host."  Dorothy Parker