Having only vacationed in Seattle, I can't speak to where to buy stuff locally. But I second megalowmatt on the Topeak Explorer Rack. The MTX bags and baskets are great. I've got the basket, into which I toss my backpack most days and use for stops at the grocery store on the way home, and I love it. The handle could use a little reinforcement, but it's my only minor complaint. Gotten a number of compliments on it, too.

As I said, I use a backpack for my commuting basics, including my laptop, a change of clothes and shoes, and a towel. I leave baby wipes in my desk at work and grab them before I head to the restroom to change. I have the usual patch kit and tools in an under saddle bag.