In most cases, the available office spaces and buildings are easily catalogued by the real estate professionals.  They maintain inventory of listings from broker to broker, and some cities enjoy the availability of a single clearinghouse of this information. 

Some resources like LoopNet and CoStar allow ad hoc membership for a fee allowing the tenant to view this information as well. So it is reasonable to say that your access to office space and buildings will not be limited from one real estate professional to another.  However, be sure to ask specifically how he or she does maintain a monitor of these to assure yourself that as spaces come on the market from time to time, especially if your lease project will take some time, you will have immediate access to such information.