For my own cultists (I am verging on 300 now!) I have used a lot from DV (Dark Vengeance) then mixed those with Imperial Guard (Cheap stuff from eBay mostly as rescue jobs) So far the mix is:
Cultists proper
Catachans
Steel Legion
Cadians
Storm Troopers
Then I mixed 60 Zombies into that mix to create mutated cultists and Typhus needs Zombies after all.
finally I have about 30 odd Necromunda gangers that make awesome cultists.
The result is a massive horde of cultists of varying walks of life that all look great and mixed. I use them as stragglers and deserters (to explain the sometimes uniformed appearance of the guard)
I either use them as Traitor Guard (full
IG list) or as Cultists.