Thus, if you're just starting out and wish to avoid learning other parts of the stack, you may run into trouble finding work. In my experience, when a Freelancer needs to outsource something, it's because he or she needs a specialist in that area.
If you're going to specialize in nothing but the frontend, you're going to need to specialize in it, which means you're going to need to be rally good, write really clean, W3C-valid HTML code that works in every browser, and have a deep understanding of front-end User Experience.
Even then, you need to understand at least a little server-side code. The best front-end coders I've ever worked with still knew a little PHP so that we could understand how we were going to integrate things. They weren't experts in it, but they weren't ignorant to it either; it's naive to think that you can get away with never ever knowing how the server-side works.
With that said, you have to start somewhere, and the best way for you to do that is to look for small jobs that require just minor changes to existing front-end code. This will help build your skill-set in that area while hopefully giving you some income.