You can post in campus chat or pub chat that you're looking to buy old hunt boxes! Starring the ones you are looking for can also help others see specifically the ones you're looking for as well! Other players must add you and initiate a trade in which you give them chibis/bentos (in game "currency" ) in exchange for their boxes! Hope that helps! 💙
You can ask other general game questions on this thread, or join the pimd discord server and ask in the #advice channel.