I know this post is a little old, but since Google is rolling out the custom URLs to everyone now, I thought it might be worthwhile to answer it anyway. I've written up a guide on how to get a vanity URL on Google plus. I hope this helps!
I agree with the first answer, it's going to take a while before it is widely available. Your best bet is to keep an eye out for announcements about it. Be patient and you should get what you want eventually.
As far as I know, Google is still rolling out the custom URL feature. Big brands got the first bite at the pie, and then it will trickle down. I may be wrong, but I think that you will just have to be patient.