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.
There are so many ideas on youtube and google.