The Joker costume is a fun costume for those who really get into the Halloween party season. While the ready made costumes and masks are abundant, particularly the children's sizes, it's not the direction that a costume "purist" is going to take. A store bought costume won't win you any points or contests. A nice one of a kind, unique and realistic looking Joker Heath outfit will.
The ultimate challenge in this costume is creating a good face with makeup. Some of the prosthetic scars available these days are amazing! They are easy to apply and very realistic. Most of us know someone who could do a good job with the make up if given a picture of Joker Heath as a guide. Most makeup kits give you enough materials for at least 2 so you get a practice run.
Once you've got the face off of your checklist you need to take a look at the clothing. This is where the truly dedicated shine. Finding an old purple joker coat or pants at your local thrift store is like finding gold these days! Then there's the hexagon shirt and tie and the green vest. While not impossible to find these costume props they can be scarce at the thrift stores as people are hitting them pretty hard right now. The next alternative is eBay which usually results in a successful find.
The wig shouldn't be a problem. These are readily available and you can color it green yourself. The Joker knife is a very nice touch. Cheap knives are easily found and there's plenty of plastic ones too. If you are into collecting movie memorabilia consider this: You can grab a certified replica Joker knife to use for your costume and then save it as a collectible to enjoy long after the party. Aside from an autographed Joker photo from Heath Ledger, the knife is one of the most popular and valuable collectibles to hit the scene from The Dark Knight.
If you are planning on a Joker outfit for your party this year you would be wise to get your Joker coat, pants, shirt, vest and tie early. The other props should be easily found but the clothing can be difficult to find and listings are already starting to thin out even on ebay.(Sherman)