So many players found themselves an unexpected place like the bakery or the legendary pizza shop. They would even say where did they find such place? For the Cake Shop is found at Ozette Wetlands and Gurhacia Valley. As for the Pizza Shop... I last found it in the Arca Plant while I was wandering around the place... and then I ran into the shop.
The best thing to find these shop in these confirmed areas is to keep checking for DEAD ENDS. And your missing 15th Mission is when you accept the Naura Sister's request to find their little sister in the Rioh Snowfield.
If you cleared all 15 quests in Hard Difficulty, you should get the Title, "World Protector". The Title reward is "Compress PA." You do not wish to sell or trade it because you can only get this once. JUST ONCE per game card. It has the ability to reduce PA usage time by 33% (Like 1/3).
While you are at the Cake or Pizza Shop, you should buy what they sell so you can complete your weapon gallery. It is awkward that you can use food as weapons but they usually collectables.
Also, please keep this in mind that you can only play the Bakery Quest ONCE! You can replay the quest again in Hard or Super Difficulty.
Enjoy your search.