Yeah, anyone can buy... all I know that is if you get off at the main exit at Ft Lewis, go to the opposite side of I5 from the base, there is a small section of town where you will find the army surplus store. Its painted Yellow I believe and open to the public.
Lots of time, ETS'ing soldiers will have "extra" equipment that they will take into the surplus store to get money. Its actually worth the trip from PDX to the place if you need enough mags and other items. They carry equipment, clothing and veteran items. very cool place