Ghost shot ammo :
A) Corpus - the recently deceased leave a portion of their spirit in the carcass left behind, with the carful application of a ritual containment can successfully siphon some of that essence or ectoplasm turning the spiritual into substance. Grave robbing is an offence by law and moral standing.
B) The tricker version for more ammo - Destroying/Capturing a Wraith and converting them into ammo. storytellers discretion is advised as the wraith is not going to go willingly.
&) Pure Silver Bullet casings. Spirituly the only pure substance that can hold Ectoplasm indefinatly. it has to be pure silver or the bullets degrade at the same rate of the discomposition of a corpse.