Any reputable contacts dealer is going to need your prescription, regardless of whether you need corrective lenses or not. I believe it's the law in the U.S. Even if you don't get prescription lenses, the seller is supposed to make sure you have a current prescription (check that you've had a contacts exam) before they sell to you. It's to make sure the lenses will fit the shape of your eye properly, since everybody differs somewhat. Wearing lenses that don't fit correctly will at least irritate your eyes, if they don't actually damage them.
Any seller that doesn't require proof of prescription shouldn't be trusted. It's for your health, and your eyes are one area you really don't want to mess around with.
Natural-color lenses can be bought somewhat cheaply. After I had my prescription updated, I bought green ones for about $30 (disposibles, 3-month supply). But they're not specialty lenses, just tinted. I've never heard of Sharingan lenses going for less than $100...generally the price I hear is at least $200 per lens. I believe it's because they have to be hand-painted in most cases. Plain red ones will be cheaper, but still more expensive than natural colors. Try checking at a store that sells glasses and contacts, like America's Best, and see what they have to offer. Tell them you're looking for costume lenses. Some of them will have a book of specialty contacts that you can order, and it might give you an idea for prices.
