12mm output motorgearbox

Hi I am looking to build a 4WD robot with 4 independent motorgearboxes. I have some wheels that I would like to use that have a 12mm hex hub. I need a motogearbox with high torque, and a threaded bar or a tapped hole at the end of the output shaft. Has anyone seen/heard of a motogearbox with these specifications? 

Ok, here's an update: Thanks to OddBot for pointing me towards wild thumper's motors and gearboxes, I now have some coming from pololu (Link) but I still need the adapters that will fit on a 4mm d-shaped output shaft from the gearbox onto the 12mm hex hubs on my wheels. Does anyone know of a supplier in britain? Really I need the adapters to look like This or This

 

Update:

I am making some adapters, but can somebody tell me - if I have a motor driver with a 2 amp output and I connect 2 motors to it will each motor get 1 amp? Thanks.

My robot has a letsmakerobots page!

 https://www.robotshop.com/letsmakerobots/node/38504

1/2" hex output AndyMark
http://www.andymark.com/product-p/am-0802.htm
Hope this helps.