I use 3 methods:
1. Play my own basslines on keyboard -when a simple bassline is needed.
2. Hire a bassist or play the bass guitar myself -when I want the feel and sound of a REAL bass guitar.
3. Sample a bassline and disect it -when its too complicated for me to play and sound good, or when I'm looking for a certain sounding bassline that I can't reproduce.
--dac