Concrete Planters from Recycled bottles! These can be SO expensive too... I need to find some large plastic containers... I bet you can put straws through too if you want to create drainage. I also think that you could make planters with the right size container!
Don't be afraid to approach construction sites and/or contractors and ask for waste material. Pieces of unused sewer pipe were salvaged (with permission - for free!) from a local construction site and turned into these sweet mod planters!