Hello and welcome to the forum
At L3 Ghosts (Phyllocrania paradoxa) are ready for houseflies,
see caresheet here. So at the L2-L3 instars the smaller
false stable flies are a better option, and can be purchased from Mantisplace. The larger fruit fly species Hydei are often fed until mantids molt to housefly stage (L3 for Ghosts) simply for convenience, because once FF are bought they are easy to culture new ones continuously - and are still within a appropriate feeder size until houseflies.
For fruit flies though they can be bought in smaller
culture vials (also at PetSmart or PetCo usually if local), but FF need to eat and will breed so a culture is the only option. If you notice though, the small vials cost just as much as the typical 32oz ones, so it won't save money. If you are looking for less flies from a ethical/care aspect of the fruit flies, I know some do, I can say there is seemingly nothing that can be done to improve their lot and usage. Honestly the size of the culture has little bearing on their population as if feed properly, they will over populate any container.
Also FF can be bought just as flies or
FF pupae at limited shops online, but they have to be added to a culture for feeding unless used directly as feeders when they emerge from the pupa stage.
