Yes, the quest Earrings can only be equipped to Sakura and no one else. You can obtain it by completing her Character Quest.

If you're asking whether the dressblooms are a regular feature in the store, no, they're not. In fact, I think it's a once in a blue moon kinda thing. Ever since the release up until now, that was the first time it ever appeared. Btw, with more and more 5★ girls being added (seriously, they're gonna way outnumber those 4★ girls), you're not gonna get 5★ dupes that easily plus with that 6% chance.

Don't worry too much about getting dressblooms for your 6★ girls, coz obviously they're already OP. Just equip a ring with a high ATK and you're good to go.