Hi there froglover9. usually dead leaves and bark are good food
for millipedes, ive kept mine for a while now and just collect leaves
from deciduous trees (of course this only applies if they are wild caught)

Suppliments i would use are calcium supps from zoo med and exo terra
and ive heard of vionate suppliment, its not specifically for reptiles but
there are people who use it, i dont know much about it though.