Start w/ ebags.com. Good prices/selection to get you started.
Depends on the 'look' you're going for. If you want eco/hipster vibe, lots of messenger bags to choose from: Timbuktu, Crumpler, Chrome, Rickshaw, Manhattan Portage, etc. I've used a Coach leather (ages well, looks great, but heavy), Chrome messenger, and Timbuktu computer bag (now). My migration was due mainly to the fact that I ride my bike to work sometimes and I need something durable, protective, and loud for my ride.
You don't have to spend a lot for leather either. I always dig the aged leather ones.
Here's a list:
http://www.messengers.org/resources/bags.html
