A quick Google search reveals hundreds of forums promising "Working Visio 2007 Product Keys." Websites like "Keygen-4-free.net" or "CracksEdge.com" will list strings like W2JJW-4KYDP-... etc.
If you have thousands of legacy .vsd files but no product key, you need to extract the data before abandoning Visio 2007. Visio 2007 Product Keys
If you have a legitimate copy of Visio 2007, the key can typically be found in one of these three locations: A quick Google search reveals hundreds of forums