There are always a couple of sphere sellers standing in front of focuser in assura.
Though even then i dont think there is enough to finish a shield from start.
Plus you probably would need a couple of mules with all invs to hold enough spheres.
I would sugest just sending your main char to assura and stay there a couple of days (or weeks) to get the spheres available and have access to restock.
Thats why i sugested SE to make spheres stackable to 99