Is there an option or an addin for Team Foundation Server that will enable 'find in files' style searching within the repository?
Unfortunately, as of VS 2008, not really. I'm not sure about 2010.
The issue is that the source code is not stored in a "file" on the server. It is stored as a series of rows in a few tables.
You can get power tools which has a couple extra searching features but still doesn't have "Find in File" type.
Or, if you have enough space on your local drive you can getl latest on everything and search useing "Find in File" in VS where you select a directory to search through instead of a solution/project.
It is not possible to search in the TFS repository. What I have done in the past is to create a little app that downloads everything to a server and then use Search Server to search in the files: http://www.pieterdebruin.net/2008/07/31/SearchYourVersionControlRepository.aspx