Long ago there were several USA-based fleets of civilian Hercules freighters, from new. Even Delta Airlines had some. They were used on various scheduled and ad-hoc freight flights, and were particularly common from Anchorage to outback Alaska oil and mineral exploration sites, but were expensive to operate and the market died away. In Europe at least the role has been taken by the Antonov-12, (comparatively) cheaper to charter. Engine replacements for jets stuck AOG is a continuing small, rapid response demand for such service.
I don't remember seeing a Delta herc, but certainly sawplenty of other USA based L-100's :- Transamerican/Trans International, Saturn, Southern Air Transport, possibly Flying Tigers (?) at LHR and in Europe back in the 70's-80's period.