Excuse me any of these have already been suggested.
In my own games these function as Brotherhood of Steel Knights in Advanced Combat Armour.
http://www.hfminis.co.uk/shop?product=white-ops-assault-team-1~hfa100&category=miniatures~modern-adventurers
As for NCR I use these guys. Considering that the NCR Troopers are based partly off Anzacs you can get away with using them as is, though sculpting on some armour doesn't take much. They don't have quite the sci-fi look of other miniatures suggested in this thread, but I prefer the more low-tech style.
http://www.grippingbeast.com/shop.php?CatID=304
In the case of Super Mutants these are pretty good stand ins, if you can ignore the hair (though various head options are available, and some are easier to clean up than others. They have a nice bodged together armour style that fits the aesthetic.
http://www.manticgames.com/mantic-shop/kings-of-war/ogres.html
Games Workshop's Lord of the Rings range is full of models that can be easily converted to the post apocalyptic setting.
http://www.games-workshop.com/gws/catalog/landing.jsp?catId=cat970019a&rootCatGameStyle=lotrI use a mix of Corsairs and Haradrim to represent wasteland settlers. Swap out some of the shields for pistols and bows/spears for shotguns/rifles.


Round off their helmets and add some goggles and the Uruk-hai Scouts become raiders dressed in riot gear and tattered cloth.

With some work these Morannon Orcs can be turned into Reavers from Fallout: Tactics

Cannibalistic Tribals anyone? Hunter Orcs from The Hobbit range.

But those are just what I use them for, there's plenty of other miniatures there that are ripe for converting. ^^