So what you're saying is that you want to play from the original bass
music instead of improvising from piano or guitar sheet music.
You can buy sheet music with the bass part transcribed. A cost effective
way might be to buy something like the
Bass Tab White Pages or, if
you play Beatles tunes,
The Beatles - Complete Scores.
That works. However, you're probably going to find yourself buying
a
lot of sheet music which can get expensive. My recommendation
is to buy
Guitar Pro software. It's very affordable and it's available
for Windows or Macs. It allows you to see
and print the bass scores.
The very best part is the availability of free tunes online. You can go
to the
Ultimate Guitar Tabs Archives and find literally tens of
thousands of tunes. You download the tunes for free, print them out
and you're on your way.