Do It Yourself
There are many ways you can lessen your carbon footprint and live a little greener, here are a few hints to help with your own rainwater collection system.
Remember to size your system based on the amount of irrigating that you need. Check out the calculations we used to determine the supply of water we would collect based upon our roof size and the demand we would have based on the amount of garden space we needed to irrigate.
Another major consideration is stagnant water, make sure to use your water quickly or find a way to keep it from growing unpleasant things. Here are som ideas: dark colored barrels (prevent sunlight and algae growth), mosquito netting, chemical tablets (just make sure they won't harm your garden plants as well).
Make sure you have a way to drain your system (if there is solid water in any pipes they will burst when they freeze in the winter!)
When building your system, try to design following this pattern: Determine what you need to do with the water (gardening etc.), calculate how much water you need to meet that need, base you system on a cistern that size.
Consider these design specs: how big does it need to be, how big can it be (spacial dimensions), how will you get the water out (pump or gravity), how fast does the water need to come out, when your system fills up where does the water go (overflow). These are by no means all-inclusive but they will definitely help you get started.
Always make a detailed part list (everything you can think of) to give you an accurate parts budget.