I'd say if you want something really awesome that fits exactly what you have in mind, go custom. It's actually not any more expensive than buying pre-made, and you can pick out exactly the pictures and art that you like.
Sites like http://www.zazzle.com/custom/posters can print up custom posters. All you'd need to do is check out sites like this for art or pictures of snakes, or even sites like http://www.deviantart.com for snake art and custom design your poster.
Hope that helps.