Best search method for a boat slip?

[theerwagen]

What's the best way to find a boat slip in a good location for the best rate?
Say one is targeting keeping a boat in Annapolis, 35 feet, monohull.
Is there some nice search engine or method that works well in various locations? or is there a typical resource or best place to start in any particular area?
How can one compare berthing costs from port to port in a particular area or region?
If there is a good thread on this, please feel free to direct me to it!
thanks