I can't buy large bubble wrap in the town that I live in. I also go to the hardware store and get Styrofoam sheets.
I use this same method for speakers or any heavy, yet delicate (I think I used to know her) item.
I use a double thickness box.
I put at least 2" to 3" of Styrofoam on the bottom.
If it's an amp, I remove all tubes and number them, then I bubble wrap them individually.
If it's a combo amp, I fill the back with bubble wrap, and make sure that the tubes and footswitch and cord are securely packaged in there so that they cannot shift.
If it's a head, I will either ship the tube separately, or pack them inside if it is a large head.
I put the amp or speaker or whatever I'm shipping in a large trash bag and tape the hell out of it. This way, if the package gets wet, the cargo stays dry.
I place the item in the box. If it is a heavy amp, like a Mesa combo, I also use at least 1" of cushiony foam on the bottom.
I then cut sheets of Styrofoam to fill all sides, making sure that there is at least 2" on all sides. If there is space inside, I use peanuts, but I push them down to make sure that there is no way they can shift during shipping.
I top it off with at least 2" of Styrofoam and use a lot of tape.
I have shipped amps to Brazil, Austrailia, all over Europe, Canada, and have only had 1 get damaged. In truth, I was out of good packing materials and cut a corner that I should not have. The amp got dropped during shipping and I'm just lucky that UPS covered it to a certain extent.
I save all peanuts and use them for smaller items.
I save all bubble wrap, foam that is in a shape I can reuse, etc.
I also have a huge stash of boxes that have been flattened and are just waiting in my attic.
The last 3 speakers that I've bought came from this forum. The guys were very easy to deal with, and had the best intentions. BUT....they used packing peanuts and all 3 of the boxes had holes and large dents in them when they arrived. Luckily, nothing but the boxes were damaged.