You really only have two options: do it yourself, or get someone else to do it. As far as getting others to do it goes, 99designs is relatively cheap.

Personally, I recommend learning yourself. Rudimentary graphic design is a pretty useful skill to have. Gotta say though, it can be a little frustrating. Unlike coding, where most of your time is spent building and adding, design requires constantly trashing and starting over, so it always feels like you're not making much progress...

It took me a whole day to design the logo for Taskforce (https://www.taskforceapp.com/) for example.

I almost used it, but I found a decent logo via reddit:

* http://www.reddit.com/r/freedesign/