Specialty manga stores or comic bookshops could be a good place to start. Sometimes they have rare or specific box sets like the one you're looking for. You can also check their websites if they have an online presence. Additionally, some conventions or comic expos might have vendors selling such items.
If not under warranty, you could try to fix it yourself if the damage is minor. Maybe some glue or tape could do the trick. But for major damages, it might be best to look for a professional repair service or consider buying a new one.
It depends on the specific context and characters. Sometimes they might follow your instructions, but often there are plot twists and character decisions that complicate things.