I use booking.com but I’m sure there are many others. I don’t know who you’re flying with, but I also suggest seeing if the airline has any stopover packages as these can be very good value and often cheaper than travel sites.
Finally, do check direct with the hotel using their website and then by phone. My rooms were about 10% cheaper than using the web and I got breakfast thrown in for free.
If you’re thinking of the Marina Bay Sands, my advice is stay one, maximum two nights, to enjoy the views and use the pool which is quite spectacular. After that go to one of the many better (and cheaper) hotels Singapore has to offer.