There are several express bus operators that ply the Singapore-Cameron Highlands route. The journey takes about 9 hours and the overnight express bus leaves Singapore about 10pm and after clearing the checkpoints at Singapore and Johor, you can rest for the rest of the journey. It is a long ride but you save the hassle of checking in and out of airports and not having to pay exorbitant transfer fares from KLIA to Cameron Highlands!
Bus companies that service this route include Starmart Express, Star Qistna Express, The One Travel & Tours and UN Travel. There is a bus terminal at Tanah Rata for local long-distance buses, however for express buses from Singapore you can also choose to drop off at one of these locations: Brinchang, Copthorne Hotel, De La Ferns Cameron Highlands, Heritage Hotel and Strawberry Park Resort.
Note: If you prefer to fly from Singapore to KLIA, and then by bus from Kuala Lumpur to Cameron Highlands, there are several options including bus from TBS to Tanah Rata.
Here's a couple of websites offering bus tickets online: BusOnlineTicket.com and Easybook.com. With online booking you can compare bus fares and select timing from a wide selections of bus operators.