First off, welcome to Coscom! Let me start by saying that this is technically in the wrong subforum, and really should be split into two threads-- one in the fabric/sewing subforum for the shirt, and another in the wigs/hair subforum about achieving Korra's hairstyle. You're more likely to get better answers there.
Since I'm not from the UK, I can't really help with the fabric situation, but I can at least say that you're going to have a terrible time trying to find a completed Korra wig from a standard wig shop. If you're too scared to do the styling yourself, try the Marketplace (available when you hit 10 posts) and look for a commissioner who could do it for you. You might also get super lucky and find someone selling it second-hand, which would also work.
If you are up for styling it yourself, there are definitely tutorials out there.
I'd also check the Cartoons subforum for the ATLA/LOK thread, and skim and/or search that for info. Chances are someone else has already tackled this same thing, and it could help you!
Best of luck.
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