It was stated in the third databook. They are kinjutsu (forbidden ninjutsu), which is why no one else who was important had them.
Deidara:
[...]
"Deidara stole this technique in his quest to find the ultimate art and used it in conjunction with his Explosion Release to infuse explosive chakra into clay, creating his signature Explosive Clay."
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Kakuzu:
[...]
"In response to this treatment, a wrong levied against a shinobi who had put life and limb on the line for his village, Kakuzu came to hold a violent hatred for Takigakure as he escaped from imprisonment. In his flight from the village, Kakuzu killed the village elders, took their hearts, and fled with knowledge of the village's most prized forbidden techniques."
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