Halloween is a great time to live out your dreams of being a Martial Artist. It's also not very difficult to accomplish.
Where to get the outfit
First off, most major cities have Martial Arts supply stores somewhere nearby. You can try the yellow pages. If you don't find the store under Martial Arts supplies specifically, try under general fitness stores. These days, Martial Arts is a more common means of getting fit and thus even other fitness-type stores can carry some Martial Arts uniforms.
Also, many of the Martial Arts supply stores are online these days. Most of the Martial Arts supply stores are use to dealing with people remotely in order to fill orders for away contacting them for uniforms. After all, that's how most schools get their actual equipment and uniforms. Thus, they are well equipped to handle orders coming in.
Finally, you could get creative. Depending on what is your inspiration, you can find a variety of substitutes that can work just as well. For instance, if you like the monk look, like what Jet Li had in Forbidden Kingdom, you can look for an oversized white robe or go to a hospital supply store with something similar but all in white. Even a painter's smock synched at the waist could work.
Part of what makes to costume Martial Arts over just being any Asian person is the accessories you choose.
Obviously, the easiest way to make sure it "looks" like Martial Arts is to equip weapons. Equally obvious, many situations do not allow themselves to having such equipment. So alternatively, you can use cardboard representations or children's toys. For instance, a pirate's sword from the Pirates of the Caribbean is a saber and works for Martial Arts as well.
Another common prop are jewelry like beads for monks. Plain plastic beads can be picked up from any variety of stores like Walmart or Kmart. You can be less obvious by using fancy chains or hairpins that in the movies Martial Artist use as throwing weapons as their enemy escapes. Customize these items with symbols, like paper cranes on string or threads of beads or cloth, to make them stand out slightly. These things become great conversation pieces too. Just be prepared to "whip it out" and show how your Martial Artist might use the item.
One of the easiest accessories is a headband. In fact, if your child is looking to be the Kung Fu Panda this year, you can find a bear outfit or just some bear ears. Dress them as you might be trying to make them into a panda. Then just tie a red bandana on their head. Viola!
It's all in how you play it
The last part of the costume is how you play the role. If you just stand there, you will look like you're trying to be any Asian person, no matter how well you match the dress and accessories. So practice your kia, or the sound you hear that Martial Artists make to explode their chi into their opponents, and get into a good fighting stance and you'll be ready to take on any Halloween task.
Have fun everyone!