Making up your own bass lines isn't as hard as you might think. True, you do need to know a few nuts and bolts but, once you acquire a bit of knowledge, your creativity will blossom.
I have loads of other lessons like this but this one might also interest you:
Scroll down for the backing track and PDF.
Pop Rock Backing Track 110BPM
Chord Progression + Shapes PDF
Use these patterns as a springboard for improvising bass lines. You need to know way more than just the patterns on the PDF so be sure to learn the scales in different octaves and patterns around the fretboard.
This lesson might help with that: