Civilian clothing etc. is something you will manage in no time, and a visit to the second hand shop will help for the rare occasion where you lack something. However there are other types of gear that will take some thinking.
Take for example Hazmat suits. I mean come on - hazard suits are awesome and well if its Fallout or STALKER pretty much a must.
So how do we get a suitable alternative? First of all, check for Workman clothing shops around your area. They often have chemical/dust/acid protection suits.
If you don't have a gas mask and there is no army surplus store around, don't worry you can still find substitutes. Yeah...workman clothes/tool shops are a boon aren't they?
That spectrum covered, what about soldiers? I mean its obvious we'd have some. For camouflage clothing, I'd say again army surplus stores, airsoft stores, hunting stores...All have it as well as tactical vests, helmets and so on. A balaclava is dirt cheap and quite enough on its own if you are not supposed to be regular military. For a more sci-fi look, check out the sports goods, namely motorcycle armor.
All in all, motorcycle armor is pretty much instantly ready sci-fi soldier armor that you only need to put on. You can add some extra stuff and paintjob but thats it. Get a full set, and get it 2nd hand it probably cost you about a 100 bucks, and you don't exactly need it in pristine condition, considering what's about to happen to it.
With some creativity and you will look just like a riot Robocop or a generic sci fi trooper.