5 gallon buckets

Try any painting services that you know of or food manufacturers such as bakeries.  If all else fails and you need to purchase them, Mac's stores usually have them or can get them and I think they're only a dollar or two.

bakery dept. at grocery stores I buy three and five gallon buckets all the time from there, some places give them away others charge $.50 or $1 for the bucket and lid.
Frostings & mixes come in five gallon buckets...they usually have an abundance of them

PK casino. 

Before you go thru all this trouble, contact your county agent and discuss a product called Plateau.  As I recall, there was some research done in Colorado using it in between the tree rows.  It was not applied over the trees, it did provide good bindweeed control as well as soil residual and it seemed to be safe to most trees.
Maybe you want to google colorado, bindweed and plateau for  some research. I is used extensively in the high plains on fallow ground going to wheat and provides great bindweed control 
Good Luck

I know school is not in session but i remember when i was in grade school i worked in the kitchen and lots of things came in 5 gallon buckets. I would contact your local grade or high school.

I will ditto the bakery places for 5 gallon buckets and some smaller buckets that the frostings/carmel toppings come in.  Try the bigger grocery stores. 

Also ditto the painting contractors.   LOTSA paint comes in the 5-6 gallon buckets; they go through alot. 

also try your neighbors for kitty litter containers which are a little smaller than the 5 gallon bucket.  

if you know anyone on the drilling rigs we use 5 gal buckets everyday sometimes by the pallet load and some you wouldn't be able to use but there is alot that you could

thanks for all of the comments so far.  Probably seems like a weird/stupid question but it's for a good cause.  Have already been checking with some of the local grocery stores, need to check some more of the options.