You have to have a lot of followers and be well known in the cosplay community before you can expect to be a convention guest. Is it okay to purchase a cosplay costume instead of making it? Hot Topic is an interesting resource, because while they do carry full costumes, they also carry clothing inspired by various book/movie franchises, as well as just well, sylvie loki costume cool plus size clothing! You might get a simple ribbon, but you can also receive a small cash prize (up to $300 if you win).
