Plastic Pop Bottles Needed

If you think about it over the Thanksgiving holiday, please save your plastic pop bottles. We are making a special project next week and will need several 2 liter or smaller plastic pop bottles for filling balloons with sand. Okay, I'll tell you, but don't tell your child yet -- it's a secret! We'll be making stress balls. I already have the balloons but haven't purchased the filtered sand yet. Does anyone happen to have any?