How to find mutations related to disease for a protein?
I think I understand what you are asking so I will make a little guide to find what you need: 
NOTE  the mutations you are searching for are probably on gene level so you can search your protein on Uniprot and then use the gene name to search on DisGeNET, you typ in the gene you are searching for: 

Then you hit the search button and this will give you a result like this: 

You can see that this site uses a sort of text-mining, in other words it is searching for the gene name associated with disease in articles. The score is an indication for how good the text-mining "worked". As mentioned in the manual:


  We have developed a score to rank the gene-disease associations
  according to their level of evidence. DisGeNET gene-disease
  association score takes into account the number and type of sources
  (level of curation, model organisms), and the number of publications
  supporting the association. The score ranges from 0 to 1.


Then you have to click the browse details... in the down right corner. This will guide you to another summary page like this: 

(NOTE I omitted a lot of rows to avoid huge images in the answer, so there will be more rows in your result). Because you were interested in prostate cancer I will click on Prostatic Neoplasms: 

You can click on the 3 to obtain the 3 articles where the combination of the CYP3A4 and Prostatic Neoplasms was found. This will give you a list like: 

As you mentioned in your question 


  "where do I find such papers"


You can easily click on the PMID this will guide you to the article, for example the first article: 


  CYP3A4 polymorphisms--potential risk factors for breast and prostate
  cancer: a HuGE review(link).


I think this will help you to find the papers related to the protein + disease, you can further read the papers to find mutations which cause these diseases.