I had the same problem,....but it takes patience. EVENTUALLY, it will "pop-up" in the shop.
Cell Jr.
In Dragon Arena, use a Level 20-40 Character and battle opponents (in my experience) AT LEAST DOUBLE your own level. When you level up, select an opponent DOUBLE of your new level. EVENTUALLY, Piccolo will "Suddenly Break In". Defeat him, and Cell Jr. will become unlocked.
Example:
Start: Level 20 Gohan Vs. Level 40 Anyone
Level up: Level 22 Gohan Vs. 44 Anyone
Death is but a door, Time is just a window,...I'll be Back!!!!!