Internet Auctions like Ebay
Packaging is expensive, both for materials and the time it requires. However, poor packaging is also expensive in terms of unhappy customers and insurance claims.
Materials. Many kinds of matierials can purchased either locally or on Ebay. You will want to compare prices to see where you get a better deal. Some people are able to find retail stores like gift shops that are always throwing out packaging materials and welcome someone who wants it. I all depends on you time and interest.
The USPS provides boxes for everything shipped via Priority Mail. We frequently receive packages from "Creative" people who turn the Post Office boxes inside out and ship media mail or parcel post items that way. Not only is it illegal, it seems like a lot of work. Not recommended.
If you have a lot of one kind of item it pays to look for the optimal way of packaging the item. For an example with postcard shipping click here.
Professional booksellers frequently use B-Flute which is a roll of cardboard material which they cut to fit. A visit to www.BookThink.com and a search for "Packing Materials" should give your a wealth of useful information.