Home Depot, Walmart, USPS has some larger ones, and if you happen to hit the gocery store at the right time there are usually lots of boxes, canned goods and cereal box boxes are good sizes. If you get a roll of clear packing tape and completely cover the box in it it makes it darn near indestructible. Hard to open, too, LOL.
I've had very good luck shipping saddles via some of the private companies that ship via all methods. They boxed things up really well. I just took the saddle to them and they did the packing. I believe the last one I used was a Mailboxesetc or some such.
A good horseman doesn't have to tell anyone...the horse already knows.