Enter the right events and there are sometimes water bottles in the goodie bag at the end. Or, buy all your bike bits from the same LBS, and eventually ask them for some freebie bottles. Lots of them seem to have bottles with their logo on, and give them away free as a bit of advertising. We got 4 free with our 2 new bikes a few weeks ago, for instance, but I've had freebies before from other bike shops (and running shops) in the past.
Mike
Mike