Contact lenses are not the most used "accessory" out there since they are not for every one. For example I can't wear contact lenses because my eyes are not very watery, sometimes they itch and I have to add some lubricant eye drops to stop the burning T_T . But they are certainly a very cool accessory if you want to radically change your look. The first thing people look in a person is the eyes, so your "new" eyes sure won't be unnoticed.
Anyway contact lenses, and especially these colored ones are better to be worn the less possible because they might damage your eyes if worn for too long or unproperly or if they aren't clean enough. And anyway it's better for you if you go to your eye doctor before using them to get all the precautions.
Colored lenses are more easy to be found online or in alternative gothic shops or at shops that sell halloween or party stuff.
There are different kinds like these ones in the pick about that actually GLOW IN THE DARK!! *-*
Check out this site for more pics of contact lenses --> designerlens.co.uk
Also beware because many times the lenses might be sold ONE per package..so read well before you buy!!