The way that I did it was I bought a base costume and over time I've been replacing parts and improving it. Really it comes down to how you want to do it. If you're looking for something fast and/or cheap then getting it all as one costume would be best until you have time to wait for delivery and funds for purchasing.
Now are we talking TDK Joker or a more traditional route? If it's more traditional then we have this full costume here:
http://www.milanoo.com/The-Joker-Cosplay-Costume-p14557.html
that doesn't look half bad. However if you're going TDK route you've got more options.
Full costumes:
Hellocosplay has several items and a full outfit that looks decent, the pants might be more blue than purple but that will usually be the case.
http://www.hellocosplay.com/index.p..._result&search_in_description=1&keyword=Joker
Cosplaysky is pretty much like HC but their costume is of noticeably different quality.
http://cosplaysky.com/catalogsearch/result/?q=joker&x=0&y=0
You can try eBay where they're usually sold as full costumes and cheaper but generally less screen accurate.
http://shop.ebay.com/i.html?_nkw=joker+cosplay
Just check around most cosplay store sites and you should find a costume there. Buying it piece by piece will most likely yield more accuracy and better results.
Utopiabase has mostly everything except the pants and they seem on the higher quality side.
http://www.utopiabase.com/movie-themes/the-dark-knight.html
Magnoli Clothiers is an odd choice in that you could get everything you need there or just pieces but it will cost you. A lot.
http://store.turnbullandasser.co.uk...quard_Weave_Tie_featuring_Stairway_Cubes.html
He's got the most accurate gloves and they're actually not that expensive. If you don't go for his gloves your only other option is usually these.
http://www.amazon.com/Batman-Knight-Joker-Gloves-Adult/dp/B001DNI938
Turnbull & Asser was the official supplier of the Joker tie but they don't ship to the USA so you'll probably have to settle for something else. Bromleys who has the best suspenders however does ship to the states.
http://www.classicwardrobe.co.uk/Gr...x?query=Categories/Products/Categories/Braces
Shoes I already gave an amazon link above to a pair that work otherwise you can go magnoli or try finding another pair that you feel works. Generally something like a wingtip type of shoe will look best.
Hair you can use a wig but there aren't many quality ones or if you're hair is long enough you can dye/spray it.
War paint you can use any kind of make up you want like rubies or mehron, I personally prefer mehron since it's a better quality of grease paint.
You can get any sort of chain for the kind he wears and it should do the trick if you're not looking for accuracy.
That's about all I got, any questions and I'm sure the other members and I would be happy to help. Also check out the old costume thread that this one is a continuation of. About 10 posts down on the first page is several links and also reading through the thread couldn't hurt if you've got the time.
http://forums.superherohype.com/showthread.php?t=327886