I doubt a proofing course would be helpful.

I do a little bit of professional proofreading. I have decades of experience editing, and I get some work from a college friend of mine -- we both got journalism degrees at the same time from the same university. I went into newspapers and she went into professional proofing.

Proofing and editing are two different things, but there's enough overlap to allow me to do it.

If you think you'd like to proofread, find an agency and let them know you're interested. Ask if they'd give you some test work.

It'll be harder than you think!