How do pack a pair of large speakers

See Kr4 comments. That’s the proper way to do it. Otherwise you are screwing the buyer or just taking risks with shipping. Even professional proper shipping service and OEM packaging can still damage speakers at the hands of USPS, UPS FedEx or Freight companies. But it’s the best chance with OEM packaging.

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.

@kr4

+1

or moving companies have folks that construct packaging on the fly. They have wood and can package anything. You can have it done, just for a set of speakers... but it is expensive. For instance about seven or eight years ago I was going to send some speakers... it was over $500 to fabricate crates... and they were not large. So shipping them... was over $1,000 (crate plus fright). They were probably only 40 lbs each.

I made a couple runs at it and gave up.

You buy new packaging from the speaker manufacturer.