I agree wholeheartedly with John's comments - a worthy ambition and one that should offer a decent return for effort expended, given that much of what you will find will be new ground in recent historical research terms.
First comment is if you can get yourself to Paris for a reasonable period, the librarians at the Bibliothèque Nationale on Quai François Mitterand in the 14th have always been incredibly helpful, in my experience. You will need to apply for a Reader's Card when you get there, but that should not be any problem - a letter from your mentor or a careful explanation of what you are doing should suffice. I would recommend spending some time surfing Gallica before going to at least start narrowing the search.
Second - Steven 'Bookman' Smith recently posted a link to a series of memoirs or descriptions of Fouché and the 'secret police' - not sure if that would be exactly on point, but it might offer some clues to sources.
Lastly, Mark Urban's The Man who broke Napoleon's codes: The story of George Scovell, Faber and Faber 2001 has some useful pointers in the 'Notes on Sources' section.
Please keep us informed on progress, if you can - I, for one, would find this fascinating
Best of luck