They are stored in the database - not in the file system.
Your options are:
1) Use BIDS and deploy to the new system
2) Use a tool that will move it for you.
There is a tool available here: http://www.sqldbatips.com/showarticle.asp?ID=62
If you have to do this 'manually' without being able to use the above tool, then I would recommend the following:
From Report Manager - list the folder/reports in detail mode. From detail mode, select the option to edit the report properties. On that page, you have an option to edit/update the report. Select the option to edit - this will allow you to save the report (rdl) file to the file system.
Once you have all of the reports in a folder on the file system, open BIDS and create a new project. In that project, add existing item - from the folder you created to hold the RDL files. Repeat until you have added all reports.
Configure the project to deploy to the new report server and deploy.
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