Midi keyboards are probobaly give tah most vesatilaty with bass patterns .. The old Korg Tritons are pretty easy to use with Midi functions ect.. Although My soundcard came with a ton of vst plugins and softsynths that can be manipulated into some ill bass soundz.. Other than that try samplin minor keys and down pitching them or even guitar stringz.. I just get creative wit shit..
Another tip I would suggest to producers usin FL is timestretching your samples to fit your drum bpm.. theres a function where u can individually change pitch, mul, and time.. this makes it easy to match ur drum patterns..
Also try adjusting your individual drum instruments to different levels or using a cut filter or resonance .. (Ex. change frequency and velocity of your hi hats).. this gives more range in your drums, just expirement wit different functions 2 get best results..
Cant give away all ma secrets, hope this helps tho..