If you are looking for a larger assortment of Priority shipping boxes than your local Post Office carries, you can set up a free account at
www.USPS.com.
The boxes are free and they are shipped to your home for free. Some of the items that I use are:
Priority padded bags
Box 4 and Box 7, which are small square Priority boxes
The game box that Robert showed
Regional Rate A and Regional Rate B boxes
For some reason, these items have never been available at Post Offices and they are only available through USPS.com.
The Regional Rate boxes can save you a ton of money when you ship Priority to zones 1 through 4. You can print Regional postage from the eBay/Paypal site when you sell something on eBay.
Regional Rate pricing can be found here, along with all of the current Postal pricing -
https://pe.usps.com/text/dmm300/notice123.htm#_c078
Zone charts can be found here -
https://postcalc.usps.com/DomesticZoneChart