Several different sources. I paid for entire disk collections, and/or paid for single files which I downloaded. Here are my sources, which I copied from my credits spreadsheet:
Audio Visual Credits
archives.gov
Army Corps of Engineers Office of History
Army Engineer Magazine
Authentic History Center, The
beatsuite.com
Center of Military History
Department of Defense
Gary Swanson - veteran's interviews
Gilles Villain
Google Earth - NASA
Historylink101.com
hq.usace.army.mil
Jason Morris and the 60th Infantry Div
Misha Pavolic
National Archives
pbs.org
Prelinger Archives
Productiontrax.com
rootsweb.com
sounddogs.com
U.S. Office of War Information
Shutterstock Images
Scripps Howard
US Army Signal Corps
Naval Historical Center
US Coast Guard
Imperial War Museum
Let me know if you have further questions. 
Proud Daughter of Walter (Monday) Poniedzialek
540th Engineer Combat Regiment, 2833rd Bn, H&S Co, 4th Platoon
There's "No Bridge Too Far"