There aren’t many of them. Natasha’s past as a dancer was first mentioned in a super off-hand way in an old Daredevil issue, and has likewise been built up mostly through lines of dialogue.
There are two short stories (like, not even a full issue long) that you might want to look at, if you’re up for tracking them down:
— Captain America: Sam Wilson #7 (2015)
— Solo Avengers #7 (1988)
— and maybe Black Widow #16 (2015) if you squint
And then there are a few longer stories that deal with the ballet idea but in the context that her dance training was just brainwashing:
— Winter Soldier #6-9
— Black Widow: Homecoming #1-6
I’m sorry I don’t have better recommendations for you!