I've always picked it up by ear, sheet music, even official is often wrong and not true to the exact of the song.
It may be hard to develop at first but you will thank yourself later for developing the skill of listening to the song and pulling out the parts you want. Try first getting the general notes and worry about all the extras Paul puts in later.
Nowadays it's easier since you can take the song in question make a .wav or .mp3 of it and use an audio editor to loop the part you want to learn so it will play just that part and start again, and again, and again until it's burned in your brain
Sound Forge, Audio Pro are just a few.
Also you may want to look at MIDI as there are lots of Paul songs in MIDI some are done very professionally. With midi you can mutes all track except the one you want, bass and with programs like Cakewalk, now Sonic you can even transcribe it to sheet music.
Hope this is helpful.
Sure beats lifting a needle up and down or rewinding a cassette back and forth.