Balloon weights can be made out of material squares, tissue paper or even a balloon.
You can fill 1 balloon with rice, salt or water, then attached your helium inflated balloon/s to the balloon weight using string or ribbon.
Another way is to fill a small freezer bag or similar with 200grams of sand, rice, beans. Tie a knot so the filling can't escape. Cut out a material square or tissue paper square. Place the bag in the middle and bring up all the paper edge points together and secure tightly with ribbon at the base.