Some great resources for office space/coworking matchmaking:
The Cube is a CoWorking online community touts itself as an online water cooler. More of a "full service" community its also a good place to find space, or find people to share space.
SuiteMatch is just what it says it is - like a dating service for the workplace. They cover all of the larger cities coast to coast, and seems like a good way to find shared office space.
Rofo is an online Broker of sorts, providing free listings of your space, and free postings of your space needs. If you make a match through their service they ask for a small fee, although I was never able to find out exactly what that was reviewing the site.