. Even if no White Scar from any novel ever published has expressed a background besides being a nomad on Chogoris, there are countless White Scars whose backgrounds we just don't know.
It does give us an idea. If the, say, dozen or two marines we do
know about only come from the tribesmen and none from anywhere else...
It's probably more likely
that they recruit heavily from the tribes and few if any from the cities.
Now I'll be the first one to say that we don't know for sure. But from the information we are given (the lack of non-nomadic White Scars mentioned and the types of people we know Space Marines like to recruit from) it does suggest that the tribesmen would be the primary recruiting pool.
Also when looking at the Index Astartes, it does mention that (at least post-Heresy), "The Stormseers of the White Scars venture down into the steppes every ten summers to obesrve the tribes and their battles, picking the best and bravest warriors and returning them to Quan Zhou to become Space Marines."
Once of their initiation trials is to travel to the pyre-tombs of dead White Scars which is a "great pilgrimage for young tribal
And another section of the entry states, "However once a warrior has been chosen to join the White Scars, his tribal allegiance
is replaced with loyalty to the Great Ghan and the Chapter. Warriors from different tribes are therefore mixed with one another in squads to break up individual tribal loyalties."
There seems to be a strong assumption that every single White Scar is a tribesman. There's no "If an aspirant is a tribes then..." "All tribesmen will then have to..." It just flats out states that every warrior that becomes a White Scar has tribal allegiance.
I mean, potentially, the White Scars may have drawn recruits from the cities during the Heresy, but for one reason or another they decided to stop post-Heresy--perhaps they found the recruits unsatisfactory?
While the matter isn't quite definitively settled (when are they going to release a Forgeworld book on the White Scars!?!), I would say it is more likely they recruited heavily--if not exclusively--from the tribes than not.