There's a good chinese import site called milanoo. It's wholesale, so the prices are really low. I've ordered from there before, and it's great quality, authentic lolita. I got my order pretty fast, too, but if you want it before halloween you should order as soon as you can.
Just go to www.milanoo.com and click on "Lolita Clothing." They have dresses, blouses, skirts, bloomers, and footwear. You can also click on a certain price range that you want to look at. If you sign up to receive emails from them, you get info about any sales they have. They have pretty good sales sometimes.

As for what you need for the look:
1. skirt and blouse, or a dress.
2. matching thigh or knee lenth socks (mine are black and white striped thigh highs)
3. dressy shoes (if u can't spend the money right now, some cheap black mary janes would do. or any black heels u have, if they match. I just wear converse with mine)
4. whatever other accessories u want. necklaces, bracelts, chokers, etc. lacy headbands are common in lolita, too. anything that u like that matches with the rest of what ur wearing. u could try hot topic or claire's.

I wouldn't worry about a wig, unless you can find a good one. Just straightening your hair with a flat iron would be good. I spray a little tre semme heat protection spray on my hair before I straighten it. When i finish, I spray a little Aussie leave in conditioner on my hands and then comb it through my hair with my fingers. Then just comb it straight and neat. Or you could curl it if you want, with straight bangs, but I'm not sure how to perfect that look.