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What's the most convenient/accurate to find entry level salary info for different federal agencies?

Go on to a site like https://www.fedsdatacenter.com, pick the agency, job title and office location (if applicable) and sort by grade. The salaries for the lowest grades in that job position will give you an idea of entry level salaries.

Starting salary is almost always going to be GS - 11 straight out of law school. You can look up what that is + locality pretty easily. You can also look up salaries of individuals at different agencies, as already explained above.

I'm an entry-level attorney in the federal government. Almost all federal attorneys, with some exceptions (for example, the SEC has attorneys on its own special scale due to the nature of their work), start off at GS-11 or GS-12 and ladder to GS-14 and beyond after that. Aside from that, if you want numbers, just check out OPM's matrices for what GS-11+ looks like depending on where you are.