Harris Cyclery carries both gear-cable housing - 4mm - and brake-cable housing - 5mm. Made by Jagwire, which is Shimano, it comes in quite a few colors. I'd suggest blue to offset the yellow bike.

http://harriscyclery.net/itemdetails.cfm?ID=947

I hope you either have a pair of real bicycle cable-cutters, or can get your LBS to cut it to size for you.