These space-savers are grown in Japan, and were even featured in an episode of The Simpsons. The BBC describes how they are made:
To make it happen, farmers grew the melons in glass boxes and the fruit then naturally assumed the same shape. Today the cuboid watermelons are hand-picked and shipped all over Japan.
You can buy yours in at Urban Fare in Vancouver, which touts itself as the only place in North America which sells them. Have you seen a square watermelon in NYC? Let us know where -- send an email to food (at) gothamist.com.