If you can't cut into the drainpipes, maybe you could put guttering on a garden shed? We have two waterbutts attached to our greenhouse, and they give a plentiful supply of water. If you have the space, then, you can link waterbutts together to collect even more. It is quite an expense to start with, but well worth it, especially if you are on a water meter. We also have seven butts attached to our house, and never need to use mains water now.