Bicycle Habitat, Sids, Toga.
The larger shops have them but they are REALLY expensive at all.

I did well here
http://www.bikebagshop.com/

The key issues are
a) Heal clearance
b) Waterproof vs water repellant
c) Size
d) Can you strap items on top
e) How easily they remove incase you want to use it as a bag into work etc

You Do not have to buy a pair, but it helps.