I think The Order is mostly culled from Liberty. Liberty being based on the USA, Order members could be any ethnicity, race, or religion. Honestly never noticed they had an accent before, you sure?
In addition, I feel from events in the Single Player campaign that The Order also recruits members from everywhere in Sirus, from all Houses and factions.
Mostly libertonian, but they've got members from every house, and probably even some corsairs and other criminals there. I doubt any outcasts belong to the order, since they 'worship' nomads.
The order itself isnt based on a particular current-world group though. Their ship designs and names are egyptian, other than that they're not related to anything currently in existence - they're just the guys that fight the aliens.
The Order was founded by the Liberty Government a long time ago, and the majority of their recruits do come from Liberty, as well as a lot of their covert funding and technology. The Zoners also allow them the use of their shipyards, freeports etc, and through the Zoners they acquire more recruits, supplies and information.
But yeah, for the most part a lot of them are from Liberty (which is also the most populated house).
The story behing my Order guy is Swiss, his ancestors came from the Rheinland. Then I know we have some Kusarians and a few Bretonians. The rest are Libertonians so we have some of all.