EDIT: Just realized I completely misread your post+title because I'm tired as heck; I thought you were asking how to get good with characters outside of playing with them, rather than how to get good against them without opportunities to play them. Everything below will obviously be a little bit off because of that, obviously, but my suggestions are sort of broad and can be applied to how to learn/improve upon any aspect of how you play Smash, so it can still sort of help I guess.
You can never get good with anything if you don't practice it. But the most effective way to do so is to, as Teneban stated, to study/observe different things other players use at pro/higher-level, and then try to implement what you see into your play.
Better yet, before watching those vids, go checkout the respective characters' subforums and learn the old fashion way - read. Guide threads, MU threads, and Hitbox/Frame Data threads aren't considered important and often pinned without good reason. Read through those whenever you have spare time (if you're willing to put in the effort), and then look out for the things you read about in the pro videos. Then, as mentioned earlier, keep all of that knowledge from reading and examples from videos in mind whenever you get the chance to play. There's no need for a sudden, complete overhaul of your playstyle - take things slow, and tackle one issue at a time over days/weeks. Mentally note how you can change every little/big aspect of your play for the better, write them down in the order you'll focus on them in the near/far future, and keep taking these gradual steps towards that perfect cookie cutter pro version of your character. That's at least what I'd suggest doing when not playing during your spare time.