Making a good costume is pretty easy. I went to a costume party one day as a mad scientists assistant with a pair of scrubs I stole from my mum. A lot of clothing store have stuff to use.
But on the other hand, the guy I went to that party with, his sister is a Star Wars cosplayer and she made her Jedi costume and it looks fantastic. If you put some effort in, it'll look great.
Btw, search google for it. "Cosplay Killzone Oxygenmask" should do it.