For relic hunters, artifacts of a bygone era hold some charm. But beware, Civil War munitions still kill people to this very day. I don’t care how out of date this page has gotten, there will still be injuries and death from unexploded ordinance from past wars. France tracks deaths from WWI munitions yearly, the last few years average 30 deaths per year.
If you are after relics of more modern wars, take care when digging. Talk to other experienced hunters to learn your trade. I’ve heard Russian metal detectorists scrape dirt off targets sideways, instead of shoveling down. “Why?” you ask. Well, the partners who survived spread the word of the wrong way to do it. Remember, if you die, you won’t be around to buy my other books.
The good news, there are many primary and secondary sources to tap into for your research. The military machine also was very good about photo reconnaissance, so you will have good luck finding certain sites. So will your competition.
If you are after Civil War stuff, I’d recommend the Official Military Atlas of the Civil War. It has a collection of wonderful maps and tidbits that can even show where buildings were standing. I believe Barnes and Noble reprinted these maps in the last decade.
All other wars, from 1812 to the Gulf War are heavily documented with books, those books list other secondary and primary sources. Those sources are federal and by the FOIA (Freedom of Information Act) these sources can be obtained.
Soldiers were also quite good at publishing their diaries and memoirs, so these primary sources can be a goldmine of tips and leads.
Your local historical society and state historical society will have a list of books relevant to your area in regards to those wars. This should be your main line on getting leads.