Memorize the comebacks you like, and workshop new ones.

But also...witty comebacks aren't as helpful as you think. Like they very rarely deescalate situations or even indicate that the other person is being rude.

Like I typically use witty comebacks when I'm either trying to be silly, or I'm serious but I've given up on the person I'm talking to and I just want to make them look foolish for everyone else (which is rare).

whenever I do some message, they reply me with messages that have that sarcasm that look funny and insulting and I don't have any comebacks for them.

Say "Hey I think you're being super rude and I don't know what I did to deserve that." No comebacks needed. If they don't respond constructively, then literally just tell them to fuck off and disengage. You don't have to tolerate people stepping on you.