changing gear hub
 

I just joined the forum today.
I have a Kentex aluminium folding bike with a 3 speed Sturmey Archer gear & want to change to a gear hub with a wider range between low & high gears, since our city is very hilly. Here are my questions:
1) which gear hub will fit my bike?
2) where can I buy it?
3) how much should it cost?
thanks :) !

A Shimano Nexus 7speed hub should do the job. They are widely available from Shimano delears, but pick a proper bike shop with a good wheel-builder, since you cant buy the wheels ready made.
The alternative 7sp hub is one by SRAM, which has a wider range.

http://www.sheldonbrown.com/seven_speed.html