Super Slayer box is dropped from any slayer monster on task with a drop rate of 1/200, extra % drop rating from gear or donator ranks in-game do not affect this drop rate so it will always be 1/200.
Farming super slayer boxes to either sell in-game or open yourself is one of the best ways to make money as a starting player. the fastest way to farm Super Slayer boxes (SSB's) is to have your slayer master set to Vannaka as the tasks are the easiest and fastest, the boxes drop rate is 1/200 regardless of the slayer master.
All rewards are 1x except for bulging bags. drop rates for each item are unknown, guaranteed drops consist of 15k-50k bulging bags. rewards include:
1. 15k-50k Bulging bags
2. Skull Charm
4. Lava Gun
6. OP Key