100 of the pieces are within the Underground, while 20 are hidden in the Card Kingdom. Be sure to look wherever you can, even inside the house of a pair of skeletons. Three can be found as early as the Ruins entrance, and three can be seen in Temmie Village, located at a lily pad path from the Quiet Hub in Waterfall.
And don't worry for those who haven't unlocked the Gold Grind rooms - no puzzle pieces are hidden in there at all. And to spare you some trouble, you won't find any puzzle pieces around Hometown.
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