From your emails and my own trial-and-error, I know how hard it can be to find the right soft plastic cups. Solo® brand cups are just plain terrific for making a classic sparkleball. Their unique shape means that 50 make a gorgeous sphere. There are other brands that will work. It’s really about the shape. Big mouth, little base, slanty sides. Above are cups I’ve used with success.
For some strange reason, solo-style cups are an American thing. Sorry, rest-of-the-world.
Warning: many 9 oz cups do not work for the Original Sparkleball. If 12 of your cups doesn’t make a nice circle for the first ring, don’t despair. Switch to instructions for hard cups or any-size cups to find the right formula for the cups you have.