No one advertises their need for part-time or on-again, off-again supplemental help. That doesn't mean the need isn't there.
I know several "retired" controls engineers who fill in staffing gaps for companies on a contract basis. But to find those gigs, you need contacts within the company -- HR knows nothing about them, nor does the web.
Keep in mind that for these jobs, YOU are the one that needs to be flexible. "Part-time" to the company may mean "40 hours a week for the next two weeks only", not "10 hours a week for the next two months". The man-hours are the same, but the deadline isn't.