Try expertflyer.com, or kvstool.com. The former gives you a trial period. But note that the information you will get will look something like this:

Y9 B9 M8 N7 S7 T7 G6 Q6 K6 L6 E6 H5 V0

All you will know for certain is that you can buy 9 economy seats. You know nothing about the load factor for the flight, other than that some seats have already been sold.