We have a rather mottley collection of water butts from an office furnature manufacturer (old glue butts, I think), a lettuce farm and even some that came to the UK full of mango chutney (the smell never washes out though!)

If there's a local farm sale of any kind, try going there. I sell shrubs at York Livestock Centre and often see the big square tanks go very cheaply indeed. Wheelie bins are also often available at sales and they make great water stores.