Federal contracts are published in a handful of places as well like FedBiz.
Federally for small businesses it very much helps to be women, veteran, disabled veteran, alaskan based, or minority owned, to win work. But don't listen to haters and bad PR, you can do it if you don't fall into that category as well.
If you want to get into military or intelligence community work sometimes you need a clearance just to see the bids.
What'd I'd recommend for a lot of people is to reach out to larger, small businesses (like 300 person) and see if they have any work to sub out. Then try to go from there.
I subbed to a company that was on a large IC IDIQ and it was easy to grow under that, but that situation is unique and anecdotal.