You can get some high priced surplus gear (with really terrible customer service) from Irving Rivers in Byward Market...or JT's Military Surplus in Nepean. The Mountain Equipment Co-Op may also have some footwear or Camelbak gear that you might be interested in (located down Richmond, near Lions Park).
http://www.milsim-supply.ca/ is also located in the city, but isn't a walk-in store, you should contact them before if you wanted to organize a meet. They sell lots of airsoft parts and accessories.
http://www.ottawavalleyairsoft.ca/ if you want to look for guns that are being sold in the area by Ottawa players.
No airsoft gun store exists in the city.