I'd start looking at places like:
www.nycewheels.com
www.evdeals.com
www.largoscooters.com
I'd also consider these message boards as well as there's a wealth of information to be had:
http://groups.yahoo.com/group/power-assist/messages
http://endless-sphere.com/forums/viewforum.php?f=3
The answer depends largely on what you want to spend. If you're the 'technical' type, there are all kinds of modification paths that can be taken to make a bike meet or exceed your needs. Take a look at these places and come back and discuss what appears to look good to you. Good luck!
Just beware of EbikeHawaii here as he'll try to tell you that there's only one solution - his way. He's been run off of the two boards I posted the links to.