DARPA is probably the best-known route and funds a lot of academic and corporate research (including, for a while, me).
The DoD also has a bunch of other grant-making programs (Office of Naval Research, Congressionally-directed Medical Research Program, etc) and also labs that directly do research themselves. The Air Force has a big research center (AFRL) in Ohio; the army has one in Maryland (ARL), and the Navy has one in DC (NRL). There are loads of other sites as well: the army has a night vision research lab in New Jersey, for example, and the Navy has some marine mammal stuff on the West Coast.
A lot of this work--even the stuff done at DoD labs--winds up in open literature. By policy all of that is supposed to be publicly available, so you can browse it here https://discover.dtic.mil/products-services/.
