Some marine chapters and legions recruit from a single planet. The process of geenseed infusion also tend to morph the facial features of any recruit closer to their gene-sire primarch, and the Alpha Legion in in particular strive to have everyone look as similar as possible using surgery for extra likeness. So while it's sometime fitting for everyone to look more or less the same, for other legions/chapters, having divergent facial features is a cool touch to show the scope of their recruitment. To some extent one can simply paint bare heads in various skin-tones, but it looks a lot better if the heads are designed with different morphological features.
For my WorldEater force I've long strived to seek out heads of various ethnicities to personalize my army and to display the wide net of recruitment utilized by the sons of Angron.
This is the sources I've found so far:
http://www.dragonforge.com/Painting%20service/for%20sale/conversion_parts.htm
African
Looks like this on a marine:
https://puppetswar.eu/product.php?id_product=32
North- and South american native
(These heads are on their way out from the puppetswar stock. Presumably since they don't look very good
IMO. They do however look a lot better
IRL than in these pics)
https://puppetswar.eu/models-and-bits-46/all-bits/heads/cossack-heads.html
Eastern european / Slavic (I don't know if there is any morphological difference to speak of in 28mm though, so it might just be the distinctive haircuts

.
BEWARE, I bought a set of these but they're not well scaled for marines. Too small. They can work for scouts or smaller human figures though.
I believe a newer version is now sold, which might be better scaled for marines.
https://puppetswar.eu/models-and-bits-46/all-bits/heads/norsemen-heads.html
Northern/Scandinavian
(I guess there's an abundance of spacewolfs filling this niche as well, but they tend to have a bit too crazy haircuts for my liking)
http://www.spellcrow.com/wolves-space-knights-heads-p-234.html
Nordic/Germanic
Also, we have the dreaded Disco Marines(!)
http://www.spellcrow.com/disco-knights-heads-p-350.html
(The guys with the big afro-haircuts is perhaps a bit too lighthearted a look for many of the armies in the grimdark future, and the quality seems a bit questionable
IMO, but I included them for completeness)
And the newest source I just found thanks to a tip from a friend.
Aurel_309 on Shapeways
https://www.shapeways.com/shops/aurel-1
Here we have collections of both African and Asian heads.
I'm particularly exited by the latter, as asian heads are hard to come by in a suitable size. There's a fair but of historical models of course but those tend to be in too small or large size.
There's also caucasian heads, plus women heads of various ethnicities. Great for mixed-gender guard regiments!
And they scale well to
GW models.
Let me know if there are any more heads out there that I've missed.