You can trust ebay, but it's really if-y. Accessories are good there since I've gotten cheap items for cosplays there. Just make sure that the seller has a good rating (I don't buy from anyone who has less than 98% approval, and 98% is pushing it).
Any place will have the same problem that a lot of people turn away from ebay: sizes. In any case, I would take measurements and make sure they match up. You will still run the risk of it not fitting though. Etsy might have cosplays, but I never really looked.
Your best bet to have a cheap cosplay is make it yourself, either from scratch or modify. Sorry, but that's how it is. Cosplay is not a cheap hobby.
Post style by PhallenPixie