<p>I am working with a Sharepoint document library and I'm trying to locate the source of the document library page. I'm working on the Sharepoint server. I just can't find it, where should it be stored?</p>

<p>Thank you!</p>

Your question is not very clear...
Are you refering to the "source" code of the document library pages?
It depends if you have edited them with SharePoint Designer or not. If not they should be located under 12 hive (c:\program files\common files\microsoft shared\web server extensions\12). If any modification were done using SPD2007 the files will be stored in the content database.
...or are you refering to the "source" where the files are stored?
All the files saved in document libraries are stored in the content database as blobs in the AllUserData table.