1. If you have decided to stay with modern/tactical, then find out where your local police & fire shop. Look up "uniforms" in your yellow pages and see if there's a shop near you.
2. If you are going old school (like Golden Age of comics), you'll have to visit a dance supply store for spandex. Capes you can get in a few basic colors from store bought costumes or have one made. Or, remember Edna Mode's wise piece of advice, "No capes!"
3. Accessories: just about anywhere from a mega-mart to the Mall. I say stick with sports shops for things like knee/elbow pads and maybe gloves. Perhaps EMS (Eastern Mountain) which we have in our local malls. Maybe Foot Locker, Dick's, or Sports Authority. I was very lucky at Home Depot where I struck gold with gloves, knee pads and a plain canvas belt that I can add to later. Each thing was under $20.
4. Sunglasses - Sunglass Hut is the best for seeing a whole bunch of stuff at one visit but you'll be looking at desginer prices too. Find the right model then do an internet search for knock-offs - I mean, "replicas."