Here's what I was probably remembering as to there being nothing in the letter to Herodotus (or the other letters) from Epicurus himself. This is from A.A. Long's "Chance and Natural Law in Epicureanism." Maybe the word Bailey translates as "swerve" is something else, or maybe the meaning of this passage from Long isn't exactly saying what it seems at first glance to say. Maybe Long is referring only to a certain aspect (free will) of the swerve?
However I don't think this is the only such reference to the swerve being only in Lucretius: