@Shayuna As AndrlCh mentioned there is a shop (four, actually) on the mid level of the shopping plaza. Facing the Casino entrance you'll want to take the left staircase. The Photon shop is the vendor on the far end. The other shops are for EX Cubes, Badges (Rising, Unique Weapon, Etc.) and the various forms of Astracite (between you and me I can't stand that shopkeeper's English voice).
The first option in the Photon shop lets you trade Photon Drops for Photon Crystals, Crystals for Spheres, and Spheres for Boosters (if you ever happen to need more). There are also items for sale under that tab that can be purchased with Photon Spheres. The second option is the Photon Booster shop which sells a selection of weapons as well as a fairly large selection of multi-weapon Camos (I've made a thread in the Screenshots board offering previews of these Camos on the various weapons they can be used for).
There are also some weapons that require Photon Boosters to unlock their "Potentials" at levels 10, 20, and 30. But as AndrlCh also mentioned if you find yourself completely flooded with Boosters and just need to burn through them quickly you can just buy the weapons offered in the Booster shop and convert them into EX-Cubes or use them for enhancement/augmentation fodder.
@Stretcharach Yea, that's cool. Would probably be a while before I actually got around to doing it as I want to at least max out her current main class to not completely waste all that progress, but she hasn't even hit 50 yet so it's going to be a while.