Even if you can purchase the box and packing material, the cost will be quite high because it has to be shipped to you and most of the fost of shipping is for the volume the box takes up. It can cost many hundreds of dollars to get a set of factory boxes. If you are friendly with a shop, they may be able to supply you with an appropriately sized box. Speakers should be double boxed. Also there should be enough room to squeeze in as much foam padding and/or bubble wrap. Make sure such padding and wrap covers the entire container so that no shifting of contents will leave any part uncovered.
Do not trust a shipping store to do a good job packing your item for you. I’ve seen horribly mangled gear that was barely protected shipped by such stores.
Good luck.