I have had really good experience with www.associatedbag.com
They have a lot of shipping supplies. Not sure about your particular size. But they have been great to work with on the phone, and very quick too. I needed a sample to verify my need, called on Friday, it was on my porch Monday. I have compared prices, and they were the cheapest too. Another option is your local U-haul should have these boxes they call "mini-wardrobe", which are quite large and could easily hold a large speaker if packed appropriately. They are around $7 each.
They have a lot of shipping supplies. Not sure about your particular size. But they have been great to work with on the phone, and very quick too. I needed a sample to verify my need, called on Friday, it was on my porch Monday. I have compared prices, and they were the cheapest too. Another option is your local U-haul should have these boxes they call "mini-wardrobe", which are quite large and could easily hold a large speaker if packed appropriately. They are around $7 each.