I use one and only one fruity loops instrument for all my sounds - the sampler. I think its as simple as Insert Instrument->Sampler, then there's a button on the instrument that lets you search for and load a file. Fruity Loops comes with a small selection, including some more realistic ones. From there, find out what formats the sampler will support, and go hunting online.
Alright, I lied, I do use their kick drum synth, too. But mostly just th sampler...