My research skills were drilled into me well before Google ever came along, so I tend to be pretty old fashioned in my approach. The library is still a vital resource for me, so I recommend hitting up the Library of Congress for Field Manuals. They have a large selection of material, including materials on enemy equipment. It can be costly to request photocopies, but if your local library (particularly college libraries) participates in Inter-Library Loan programs, you can "order" a particular manual in print or microfiche for delivery. Try the link posted below for details:
http://www.loc.gov/rr/scitech/SciRefGuides/technicalmanuals4.html