People dance at a ball, but a “crystal ball” pun is:

Q: Where do fortune tellers dance?
A: At the crystal ball.

The riddle has been cited in print since at least 1984.


Google Books
The Zaniest Riddle Book in the World
By Joseph Rosenbloom
New York, NY: Sterling Publishing Company, Inc.
1984
Pg. 26:
Where do fortune tellers dance?
At the crystal ball.

Google Books
Swine Lake:
Music & Dance Riddles
By Charles Keller
Englewood Cliffs, NJ: Prentice-Hall
1985
Pg. ?:
What do you call a fortune tellers dance?
A crystal ball.

Google Books
Nate the Great Goes Down in the Dumps
By Marjorie Weinman Sharmat
New York, NY: Yearling
1989
Pg. 67:
Q: Where do fortune-tellers dance?
A. At the crystal ball.

Google Groups: Humor Club
Mini Ha - Ha’ s ! ! - R.K.
R.K.
6/29/06
(...)
Where do fortune tellers dance ?
Answer :- At the crystal ball.

Google Groups: HasanShabbir
ONE LINE QUESTION &ANSWER [not serious]
Hasan Shabbir
9/3/06
(...)
Where do fortune tellers dance?
At the crystal ball.

5 September 2006, The Daily Mirror (London, UK), “Greg’s giggle” edited by Fiona Parker, pg. 37:
Q: WHERE do fortune tellers dance?’
A: AT the crystal ball.

Twitter
NotSoBright.com‏
@NotSoBright
Where do fortune tellers dance? At the crystal ball.
8:00 AM - 7 Mar 2009

Google Books
Funny Fun Jokes 2015
By Peter Crumpton
PeteyRF Creative
2015
Pg. 15:
Where do fortune tellers dance?
At the crystal ball.

Twitter
Midlands Arts Centre (MAC)‏Verified account
@mac_birmingham
Where do fortune tellers dance? At the crystal ball.
#BadJokeFriday
8:55 AM - 19 Jan 2018