I'm going to ask my ex girlfriend for her cousins phone number
Why do you want her cousin's phone number?
Because I need to have access to every part of her life
You guys have ended your relationship, so you shouldn't poke your nose into her affairs. This will only invite conflict into both of your lives and the time spent keeping tabs on her will be mostly wasted.
Why?
If your relationship is over you need to move on. It is stalker lever creepy to keep tabs on her.