There's a few resources out there for smaller companies to try to get into defense, mainly networks and incubators. [1]
But you'll also see SBIR/STTR funding topics (program run by the Small Business Administration) [2]
You can also try checking out some of the bigger Defense contractors, some have incubator programs and are looking to expand their small business ecosystem for subcontracts, part of that includes funding. (Disclaimer, I work here) [3]