Make a list of all companies in the areas you want to work in the field that you're interested then call/email them if you can't find their job listings. Most likely, you will have to apply through their website. If you're looking for government employers, look on their local/city/state website or usajobs.
A lot of companies require online applications, not even emails. You might end up applying to a lot and still only hear from a few. Keep in mind that many, especially government ones, could take 3 months to hear back. I had a job for 6 months and MA agencies were calling for interviews.
In addition to your online applications, hit up friends, school/professional colleagues to see what's going on with their companies. Attend local association meetings. Most of them have a monthly or quarterly lunch or some other gathering. I had professor offer to send requests for me when I was right out of school. It made things easier having someone they knew inquire for me. Point is, professors are good resources.
Make sure your resume is built for the job you're looking for and reflects keywords found in the job descriptions that interest you. Online applications filter resumes. And for government ones, make sure the resume has the other items they require. For example they often ask that each job shows the length of time and how many hours per week... things like that.
I've had loads of recruiters on linkedin contact me and asking if I'm looking for new opportunities. But when I was looking, they never responded back to me. I wouldn't spend much time on recruiters unless they're the ones reaching out.
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