Combat-ish boots are everywhere! Hit up the mall. Hot Topic. Sears, even. Macy's. Thrift stores. Plato's Closet, at least the ones I've been to, ALWAYS has some combaty boots up for resale. I'd recommend against it for ethical reasons, but I have a friend that's gotten great looking boots from WAL MART, of all places, for cheap.
I realize price may be an issue, but, oh man. I *HIGHLY* recommend dishing out 50 bucks or so for a rocking pair of boots that you can wear all the time. For Halloween two years ago, I strolled into Sears and came out with these babies, sadly not at all on sale, for $60:


and they have served me well since. They are fabulous. I've put them through tons of punishment and they have treated me well and look great. You can walk in them, run in them, kick in them, no problem! With stockings, thigh highs, or just normal socks? With skirts, dresses, or, if that's not your thing, just under your pants, mostly hidden?! Pseudo combat boots can do it all. If NOTHING else, you'll probably find ways to wear and rewear them for Halloween after Halloween. So many costumes are improved by a pair of faux combat boots. But, seriously--get a comfy pair, not too much of a platform, and you can do anything in them. I can sprint in mine--not as well as I can in sneakers, but impressively close.
Fake combat boots beat real ones any day of the week, because although you can play around with the laces, you don't need to tighten them to put them on. There's a zipper up the side! And no one can tell there's a zipper unless they are WAY too close to your feet! It's great. Real combat boots are such a pain to get on and off.
You can always find some on line for pretty cheap; if you want to go less comfy and more goth girly, demonia shoes is a good place to go. Here's some on amazon:
http://www.amazon.com/gp/search/ref=pd_sl_aw_flat-hi_shoesinternal_16535190_1/?node=1040668&field-keywords=demonia%20shoesAlthough you can find most of them cheaper by searching on the internet in various places. I got a really good price (like half of what they usually go for) from somewhere for halloween last year.
Anyway, sorry for rambling on, but
i love shoes. And the boot cover slip on things you can get from costume shops always look so tacky. And I firmly believe every girl should have a pair of sexy but useful combat boots in her closet.