Fred - you don't actually need jackboots, which are long and unwieldy for the most part. Any old army boot or work boot, preferably black, will do. The trick is to blouse your pants leg and tuck them into the boot, so it gives you a military look. Cargo pants with extra armyish-type pockets are best for this. You could have the same boots on, and if your pants are creased and hanging down to the laces, nobody really minds. Forget the utility of keeping your pants cuff out of the mud when you're marching through corpses of the unwashed masses, the blousing of the pants leg is what really pisses people off.

But "blouse-legged thug" just doesn't sound as menacing as "Jack-booted thug."