If you're going for a LOTR character these are the obvious choice, but they work great for any evil spawn costume.

Costume teeth by Dental Distortions are created using the highest quality dental materials available.  This style includes both upper and lower teeth.

All of Dental Distortions teeth products are designed to cover the front of your teeth only. The thin design of our products minimize the negative effect on your speech so your speech remains natural.

See below for more information and see how they work.

First photo:  This is a top view of the teeth after they have been customized.  Notice that there is no material that goes behind your teeth to impair speech.

Second Photo: This is a view of the back of the teeth after they have been customized.  The customizing material makes an impression of your teeth for a perfect fit.