@VI-Lycan-Lupgarou The Excube and Photon Drop Shops are on the second floor of the Shopping Plaza; in the central part of the Shopping Plaza, face toward the Casino entrance, and take the stairs that go up toward the left, and you will see a line of vendors.
You can buy Mini Food Devices from the Personal Shops, but they tend to be a little overpriced, though Photon Spheres (the item you use to buy the Mini Food Devices) tend to be a little more affordable; that said, as you level up, Photon Spheres will become more common, especially once you can run Extreme Quests.
Level Down Devices cannot be bought from the Personal Shops, so you need Excubes; that said, Excubes become fairly common when you play at higher difficulties, with you being able to trade junk 10*+ Weapons and Units for them via the Swap Shop.