I bought tickets directly from the site:
http://www.malaysiangp.com.my/
There you also have a list of the travel agents who are official distributors in Singapore.
Haven't yet decided how to get there (bus, train, plane or rental); need to have a look what best suits for my travel times; and what the costs would be.
I never been to Sepang for F1; but if I correctly undestood during the grand prix days, there is shuttle bus service from the Airport. There is alsoa direct and quite quick train connection from KL Sentral. Therefore booking a hotel in the city should not be a problem.
Of course if there's someone whose been there earlier years, tips would be great.