You could go in the plugin request section of bukkit.org and describe your plugin there, then someone might make it for you. Make sure to read the thread format so you don't get in trouble by moderators.
My logo was custom made by an artist. I told him what I wanted and he made it for around $25-30 if I remember correctly. You can find artists across several minecraft forums that would make you a custom logo, but they usually charge for money (at least the good artists).