To find a archen in both versions, you need to go to the Relic castle and talk to the woman backbacker on the first floor and talk to here and she while ask you to take a fossil from her: the cover fossil which is titorga and a plume fossil is archen.
Then fly to Nacrene city, and go to the gym and talk to the lady in blue on the right and she will resurrect the plume (cover) fossil into archen or titorga.
At the Relic Castle, you need to take the Plume Fossil. If you take the other Fossil you cannot get Archen.
You must revive its fossil.
In the first bit of relic castle talk to the woman she gives you the choice of plume(archen) or cover(tirtouga) fossil
i think you can only find it in Pokemon black if you have white like i do tuff numbs.
Archen evolves into Archeops at Lv. 37.
Archen evolves into Archopes at level 37 in Pokemon Black, hope this helps :)
archen
Archen doesn't evolve from a stone. But you can evolve him at level 37.
37
There is a lady with a backpack on she gives you either a plume fossil(Archen) or a cover fossil(tirtouga)
archeops at lvl 37
tirtouga and archen which you get from a trainer petillil which you trade at nacrene city but you need a cottonee and victini but you need a liberty pass which you get from a mystery gift