If you have replied to their inquiry and they have not responded, it might be a sign that they are no longer interested. Or they could be scammers or tire-kickers. Or they may be still thinking it over and all they need is a call from you to prompt them to make a visit. One call should be enough; tell them you are following up with their inquiry and now that they have had a chance to think, do they have further questions or wish to schedule a visit?
Now finding their number is another matter. If they called, you have caller ID to check. If not and you have their name, try the good old White Pages book or one of the internet phone search sites. If they use only a cell phone, there is no cell phone directory of which I am aware and you may be SOL.