First you should know that often when peoople talk about Khao Sok, they talk about its lake, the Chiew Lan Lake and its sublime landscapes that is reminiscent of Halong Bay in Vietnam but do not confuse the lake with what I call the 'Land Entrance' of Khao Sok National Park (where local buses and passenger minivans from Surat Thani, Khao Lak, Phuket or Krabi will drop you), both are quite distant. Access to the lake is possible from Ratchaprapha Dam. The 2 days / 1 night excursion with one night on a floating bungalow on the lake is often one of the most beautiful memories of travelers during their stay in Thailand. And to organize it, you can contact one floating hotel but the most practical way is to organize this with a local agency like Khao Sok Lake Tour. Other agencies offer it obviously, I'm talking about this one because I know they are reliable and their customers are satisfied. Otherwise you can also go to the ''land entrance'' of the park and spend a night there after having booked the 2 days / 1 night on the lake excursion for the next day at one of the travel agencies in the street or directly at your hotel. It cost 2500 THB per person. The bungalows on the lake will be much more basic, without air conditioning and without electricity except 2-3 hours per night but you will not find cheaper and except a very basic accommodation but in a beautiful setting, the rest of the excursion will be equivalent. Note that agencies also organize survival treks in the jungle around the lake and it is also possible to spend 2 days in immersion with a Khao Sok local family.
The best time to come to Khao Sok is from January to April. November is particularly rainy and therefore not a good time to come to Khao Sok.