When searching for an available domain name through a domain registration service there is the risk that your chosen url will be snatched up by a domainer if you wait to purchase it. Domainers are those individuals or companies who buy and park urls with the hope that others wanting a specific url will pay a premium for them. Many domain registration companies track url searches on their site, and sell that information to domainers.
Here is a way you can find out whether a domain is available without domainers knowing that you are interested in a specific url. It gives you a little time to think carefully about which names you prefer.
On your Mac, go into Applications/Utilities/Network Utility.app.
Launch the Network Utility application, and select the Whois tab.
In the first input textbox, type in the domain name you are searching for. Make sure whois.internic.net is selected in the dropdown list of the second textbox. Click on the blue Whois button (on the right) to start the search. This search may take a minute or so.
The results will indicate whether the domain name you were searching for is available to be registered, or if someone else has already registered it.