http://www.halkidikinet.gr

A very nice place to start your search considering anything about Halkidiki is:

http://www.halkidikinet.gr

There you will find all things about accomodation, tours and services in Halkidiki.
It's a very nice source about the history of the area with detailed description about every city and town of Halkidiki. The site is costantly updating so check back often for new information.

You can also check the following sites of the same company for more specific information about Sithonia and the Town of Neos Marmaras:

http://www.sithonianet.gr
http://neosmarmarasnet.gr