You know, when Orochimaru protected her against some guy?
I'm not having any luck finding the specific episode that you're talking about (if it even exists). What I did find is a reference to the Third Databook (pages 66 and 67), where it explains how the two met...
After the village was burned down and everyone was killed, [Karin] was approached by Orochimaru, who asked her how she survived, leading him to bring her back to Otogakure.
I'm guessing at this point that there was maybe a small flashback in one episode somewhere, but if there is I can't find a clip anywhere. Hopefully this helps at least somewhat! Also, here's a link to where I found this information!
It's from Naruto Shippuuden Episode 431: To See That Smile, Just One More Time.
This one is from the run of Infinite Tsukuyomi fillers that aired during the Fourth Shinobi World War arc. Notably, unlike the previous episodes in this set of fillers (mainly just fairly lighthearted happenings inside the Infinite Tsukuyomi dream of various characters), Karin's episode here is basically a rather sombre flashback to her past.
(This was a sad episode. Poor Karin...)