Party balloon decorations are not particularly difficult to find. You can find them at "beach parties" spotted all over the map. It was already necessary to find the beach parties for the very first challenge of the 14 Days of Summer event, so if you have already done this one you already know the locations of the balloons. Otherwise, here is the map with all the information you need.
Warning: a group of three popped balloons only counts as one popped balloon. You can find between 2 and 3 groups of balloons at each beach parties. Visiting only 2 of them should be enough to complete the challenge.
