the
right cup
Basic sparkleball instructions work only with this 9 oz cup. It's about 3" tall. The top is much larger around than the base. The package should say "tumbler" or "old-fashioned." Solo is one brand, but there are others. If it says 9 oz it should work.
There
are hard and soft 9 oz tumblers. Hard ones are shinier but crack easily.
Soft ones have a lip at the rim and don't sparkle quite as much. All make
toxic fumes when melted. For a no-melt
sparkleball using stapler or hole-puncher, you must use soft
cups.
Where can you find cups? At grocery and drug stores, restaurant and party supply stores. Manola in Chicago orders hers from Ebay. Andrew in Missouri found unusual cups (printed with American flags) in a card shop. You can also order cups online.
Color cups are fun if you can find them. The don't change the lighted sparkleball much but they do look pretty in the daylight.
No 9oz cups? Don't despair. You can modify the basic sparkleball instructions for any size plastic cup.