Gardeners report that free buckets are available from Burger King and Chick-fil-A ("pickle buckets"). From Lary Hall's Facebook page https://www.facebook.com/groups/144745362329424:
"Drop by Burger King and ask for their 5 gal pickle buckets. Chick fil a in your area they will give you pickle buckets too."
One user has a photo but you need to be a FB group member to see it: "The manager of Chick-fil-A to take all the 5 gallons buckets I wanted. He had about 50 and thanked me for taking them." http://buff.ly/1lCHLgR
Buckets can be found on Craigslist. One FB group user reported: "a man that owns a restaurant has hundreds. Sells for 1.50 with lids."
Another user: "Lowes has food grade for 3.75 if you get more than 5 you get the contractors price..check the bakeries I got some for free and from another for 1.00"
Rain Gutter Grow System Group Page, 04/2014 http://buff.ly/QVbQuU