Where Do Recruiters Source Job Openings?

As an IT contractor, I often notice that as soon as a position opens up at any government department, I’m flooded with LinkedIn messages from recruiters, both in Australia and abroad.

I’m considering starting my own recruitment agency, but I’m curious: where do recruiters typically find these job openings?

It seems like there must be a specific website or platform where these government roles are advertised. Any insights would be appreciated!