I just borrow the kids' used softdrink PET bottles for a while. No need to wash them out or anything, just rinse out the few leftover drops of Coke or 7-Up firs

I just borrow the kids' used softdrink PET bottles for a while. No need to wash them out or anything, just rinse out the few leftover drops of Coke or 7-Up first time you fill it up, and switch to a fresh one after a while, let them recycle the old one.