Are you looking to do 100% street or do you want a dual sport 90/10 or 50/50? (street/dirt)
Zero says your tires are...
Front Tire 20 x 3.0 inches
Rear Tire 17 x 3.5 inches
http://www.zeromotorcycles.com/zero-x-specs.phpThe 17 should be easy to find. It is the standard supermoto rim size. The 20 might be difficult.
I took a look around here...
http://www.motorcycle-superstore.com/2/29/DV/Dirt-Bike-Motocross-Tires.aspxCould not find much.
http://www.google.com/search?hl=en&rlz=1B3GGGL_enUS306US306&q=300-20+motorcycle+tire&btnG=SearchTurns up a lot of tires for antiques.
I'll keep looking.