Michael's frame shop. BUT you need to go ask in advance and be willing to make a second trip the day they call you to collect. The biggest boxes they get are 42" x 62" x 2".
They both get large boxes and keep sheets of cardboard on hand for storing art. It depends on the store and the framer, but I give away boxes to people who ask nicely. Frame shops take delivery of their boxes Monday through Friday, no weekends. In a busy store they will get stuff every day. They break down the boxes as soon as they are empty unless we have a person waiting on them. I frequently have notes at work "please call Mrs. Smith when you unpack today, she needs 2 boxes".