Everyone goes to their eye doctor to get their first pair. It's the safe thing to do! ^^
Only start buying lenses on-line once you know what you are doing and what kind you are able to wear. Never buy used lenses from other cosplayers and never share them with friends!
There are many different brands out there so you have to research well to make sure you are getting good quality as well as the colour/pattern you want.
It's not just finding a cool website with shiney looking usually photoshopped pics, it's finding the manufacturers brand name of the actual lens and looking it up on lots of different sites and youtube reviews to make sure it really is the colour/pattern you want and that it will come in the base curve and perscription you need it in. Also read reviews of the website customer service that are posted on places like cosplay.com to make sure you won't get ripped off when you do order them.
best of luck with your cosplays!