SCP Foundation

And I don't mean people like D-Class personnel. Is there a way that we have and that is confirmed to be canon in which the Foundation actually finds and hires people to be of higher security clearance levels and such?

As is usual, "there is no canon." So whatever you think the Foundation does to hire people is what they do.

From my own personal beliefs, they hire promising scientists early out of college, stick them in one of their front companies and as they work slowly introduce them to the world of the anomalous. They probably also hire ex-GOI members and people familiar with the world of the anomalous, such as retired anartists, reformed CI insurgents, Ex-GOC agents, etc. For non-Foundation scientists who discover anomalies, they'll probably snatch them up as they're already familiar with that specific anomaly. To me, those are the most likely options for where recruits come from.

people to be of higher security clearance levels and such?

What kind of people? Are you thinking scientists, field agents, security personnel, supervisors, office workers, janitors, etc.?

Here's my headcanon:

• scientists = recruit researchers out of college. Pay off those student loans, offer them an accelerated program towards a doctorate/other graduate degree, give them some benefits that can't be matched by other employers. Occasionally recruit doctors out of top-tier universities if someone has a particularly impressive C.V. and isn't particularly tied down to being faculty and isn't in the public eye too much.
• field agents = can recruit some out of college, but also law schools and police departments. Occasionally bring in detectives or P.I.s who aren't strongly affiliated with another group.
• security personnel = probably hire from police academies and the military, though I personally like to think that the Foundation would also pick from personal bodyguards and people who've been employed as guards already.
• supervisors = probably via internal promotion only. I doubt the Foundation would hire someone solely to manage part of the Foundation without said person having spent a long time in the environment first.
• office workers = pull from people with solid resumes and few personal connections to people/companies/locations.
• janitors = ditto above.

Basically, I think the Foundation would look for three things: prior information (because the Foundation loves to have records on things, and someone who has a resume, a couple professional contacts like former/current bosses, and/or a C.V. would make for an easier time determining skillset and work ethic than someone with no verifiable background whatsoever), prior experience (the Foundation's pretty top-notch in terms of expectations for the employees, so they don't want to have to explain basics if it can be avoided), and lack of strong connections (because it's a physical and informational safety liability if someone is more loyal to another company/group over the Foundation. Personal connections are fine, because then the Foundation can sweeten the deal by providing accommodations for family like health care, financial assistance, etc.)

But then, that's my headcanon.

There is the Laundry way: hire people who stumble onto the paranormal world, especially people who are smart enough to have gotten themselves in trouble in the first place.