I am using the Pig Iron Kolony militia heads but that'll probably never fly in tournies.
Here's what that looks like if you're curious:
Within
GW ranges, the Scion and Skitari are probably your best bet, though the Skitari hoods might look a little strange (and I suspect you'll need to remove the collar from the torso bit). Scout heads always looked a bit big, but might work if you don't mind no headgear, and last I tried same seemed to go for the Catachan heads on Cadian bodies.
You might be able to drum up Elysian heads, as headswaps seem fairly popular. ForgeWorld has
a set of them with respirators that I think look awful but are definitely different. If you don't want to go TOO different, there's
this from ForgeWorld that uses the standard Cadian helmets but with respirators - if you go that route let me know, I may buy some of the other bitz off you
The other humanoids in the
40k range don't quite look right, though *some* Genestealer and Chaos Cultust heads might be passible if you want a "looted" look, but it kind of clashes with the rest of the body.
I can't speak for all of them, but I've seen some people successfully use
WHFB Empire and Bretonnian heads (though with the latter, it's usually the entire body as well).