If you but it at a store in town, we always suggest that you try to pay with cash.. I personally have had no issues with that store, but others are more leery. If you have time to buy it online, then I always like to suggest
http://www.nothingbutcostumes.com/index.php?main_page=product_info&products_id=7980 as they have consistently been good to us.
Otherwise, just call up various shops and say "Hey, do you have a V for Vendetta mask in stock, how much?" if it's more than $10 or so, then just pass. Oh, and word is that those Spirit stores that go up all over the place in October are carrying them, so that could be a good start.