How do I get distortion on my fender 15B amp if it doesn't have a distortion switch or a button? I have tried using existing knobs but just cant get the rock sound I'm after.
The Fender 15B is a bass amp. Bass amps are usually focused on clean signal replication and most do little to color the sound of the instrument whereas guitar amps impart more tonal signature to the sound.
As you are looking for distortion I assume you are not using a bass as this this effect is rarely used by bassists.
If you are playing an normal 6 string guitar you will need to either buy a distortion pedal or an suitable amp.