I'm kinda new to Thailand myself, so I don't really have much I can tell you. However, there is a cosplay shop in Central World although I've never been. They have a facebook page, http://www.facebook.com/akibakiss
. I visited their photo shop near Paragon department store and they were really helpful, so maybe you could try contacting them? Oh, and speaking of Paragon, they have some costumes in their department store, although probably expensive knowing Paragon. But they also have lots of craft stuff if that's what you're looking for.
I don't know any cosplayers here, but I was told that there's some kind of cosplay event the first Saturday of every month near Paragon - either in Siam Square or near MBK. I've never been tho cos I always have to work
Anyway, hope you find what you're looking for. And, if you get some cool costumes and want to take some pictures, let me know
(I just got a lovely new studio flash that I'm dying to use...)