I get my used 5 and 7 gallon heavy duty extruded nursery containers at a local nursery...they had small trees/shrubs in them....
If you know anyone that has a landscaping business, that will be your best bet...they will probably give you a full set of the same size and color heavy duty containers to keep from fussin' with them after they complete a job...
steer clear of blowmold containers...they are very flimsy and won't last more than one season if you are careful...and by the way, if you use blowmold containers, they are so thin that if you accidentally hit one with a weed eater, it will eat a long hole in them...