Hey, welcome to cosplaying. It's awesome.
The big part of the outfit's the wig, obviously. Without the purple hair, it's just. You know. A cool outfit. There's a ton of wig sites out there, but a few big ones are Arda
, The Five Wits
, and the site that you're on right now
. There's also wigs on Ebay and stuff, but while it can be cheaper there's no guarantee on quality.
The rest depends on the outfit you choose. You might be able to thrift it, or you might have to order/make it yourself.