Are you sure that you are searching for the correct info. I just typed Halkidiki in the address bar and found hundreds of sites. Check out the world sites, not just the UK ones. Many of which lead you to others that are full of hotels and apartments. Once you have chosen a resort, do the same with the resort name and see what you get.
Having said that, we booked our holiday with a tour company in the end. Did it on-line and saved ourselves £120!!
Are very much looking forward to our first holiday in Halkidiki.