Would a retired researcher mentor a postdoc with his/her own external funding? What about one without external funding?
I'm sure you'll find people who are "retired" (from administrative duties, from chasing the next funding proposal) and still are hard at work, while others just want to stay at home and look at the lake in front of the window.

If one of them would take you on as mentoree (made up word, I know) is anybody's guess. Perhaps they retired in part to be free of having to deal with pesky students. Maybe you are so outstanding that you lure the hermit back into active work.