1. Thrift stores
If you have a simpel outfit such as a zombie, you can just buy some clothes at a second hand store, and destroy and age them yourself.
Or you can search for similar items and make the outfit. Beware, you might have to buy in normal stores as well.
If you want a ready made outfit for cheap, go and look on Ebay. Beware, if you want a china outfit, it will take a month to get to you, so order way ahead. If you still want to order on Ebay, but don't have a month left, look for stores inside your region, for ex EU only.
3. Local carnaval stores
Here you can find several stores that sell outfits than can also be worn for Halloween. Also keep an eye on stores such as Action, Zeeman, Carrefour,... As they sometimes have several outfits in their stores. At the moment, Action has a witch outfit and a skeleton dress for woman and a skeleton and a ghost outfit for men. They also have several wigs and accesories.
5. Online stores
In a lot of online stores you can find ready made outfits. Be aware you sometimes still have to add shipping and wait for them.
If you are a bit handy with a sewing machine, there is a big chance you can make your own outfit.
What are your plan for Halloween? Do you already have your outfit?