You can try Shooter's Paradise in Woodbridge, Virginia Arms in Manassas or Guns and Ammo Warehouse in Manassas. However, most shops only carry holsters for the popular guns, Glock, Sig etc. You might have a hard time finding one for a Ruger. I'd check Blackhawks website, I don't think they make a Serpa for the Ruger, though I could be mistaken.
Too many holsters, too many different guns, it's hard for a gun shop to keep a good stock on hand.