What kind of Halloween costume are you looking for? Be specific. If you're looking for a "baby pumpkin" or "adult mummy", type it in the search field. Use quotation marks around the phrase so the search engine will only bring back results with that specific phrase, and add the word "costume" (no quotes) at the end. Example: "baby pumpkin" costume.
Examine your results. You're going to get back a LOT of results. In order to narrow these down a bit, and find the best online store for your needs, you can A)evaluate each site or B)use the Google search options. Let's go with B, shall we?
Narrow your results. Clicking the "search options" link, you'll see quite a few filters: date range, what kind of results you want, related searches, even an option to kick out some of the shopping sites (or add more!).
Here's a quick example using the "baby pumpkin" costume search we've used so far. I don't want to spend a lot of time hopping from site to site, so I'm going to click the "images from the page" link, which shows me what the costumes look like right on my search results page. See how easy that is? With one click, I was able to refine my search results and make them more manageable.
More Halloween search help