I toyed around with Celestian squads all with helms...ended up just going squads of ten with unique models, helmets or not.
About 1 in 3 have helmets.
There's also a heavy weapons body with a helmet.
Get on the GW online shop and type in sisters of battle, should show you everything.
I don't think you can order helms alone, but you can minimize the non-helms, especially if you cherry pick the blisters in your local store.
There is also a Sabbat pattern helm on the Immolator sprue.
I ordered a few extra and head swap my Superiors with their stupid looking fleur de lis on their helms. Works fine. Take the head off with a jeweller's saw, just be careful not to scrath the cloak collar.