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.
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.
It depends. If you're a big manga fan and love having a collection, they can be totally worth it. You get multiple volumes in one go and can enjoy the series without having to hunt for individual issues.
It's possible for manga box sets to be fake. Sometimes, poor-quality reproductions are made and sold as the real thing. This can be difficult to spot for the average buyer.