Paul Vixie hosts a Personal Co-location Registry. If you have a personal 1U server running, say, a trading program or some such, then looking for place for it could be as easy as looking at the site.
For trading action, where you're not trading quite enough to colo right in a market data
source or broker, then setting up somewhere close to the action might be sufficient. I
ended up working for a week near Wall Street last month and was walking down Broadway and
passed by what is known as the Cunard Line Building, just up from the photogenic New York /
Wall
Street Bull. I didn't realize the significance at the time, but later found out that
Telehouse operates a hosting facility on one of the floors of the building.
Look for companies with rack space there if you want to get geographically real close to the action.