Planters - Materials?

I'd like to build some planter boxes for myself, out in the yard, but I can't afford using expensive materials to do it, like bricks or even 6x6 wood.( those beams cost $20 each! )
Do any of you have some suggestions for inexpensive materials that can be used to build nice outdoor planter boxes, maybe 1-2 feet high off the ground?
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Grassroots
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You might find some suitable waste materials at construction sites. Check with the building foreman.
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