Do you have any old notification emails? You could use the application number to link it to your current account, and then be able to download all correspondence.
Why do you need the rejection letter anyway?
I was rejected for a student permit in 2013 and applied for PR in 2018 without the rejection letter.
In the PR application I was honest and wrote "Applied for SP in summer of 2013. I no longer have any records of that application, so I am unable to recall any specific details or the reasons for refusal".
PR was approved without a problem.