You can get the latest copy of the program from the Wireshark website: http://www.wireshark.org/download.html. The website allows you to choose from among several mirrors for downloading.
A new Wireshark version will typically become available every 4-8 months.
If you want to be notified about new Wireshark releases, you should subscribe to the wireshark-announce mailing list. You will find more details in Section 1.6.5, “Mailing Lists”.