I have seen a mere one 581 offered for sale, since I had to part with mine. This is personal, eyes-on seeing, not counting on-line. That one sat a local dealer, tempting me, for a few weeks, but I did not have the cash. Looking back, I should have done some trading. It is a bit more common to see a 681 at a dealer or gun show, but just a bit. Fixed-sight L-frames are rare birds.
I managed to acquire a 4" fixed-sight GP100, without the full-lug barrel, which fills the heavy, fixed-sight DA .357 sixgun niche, though I still miss my 581, and would start a lay-away if one showed up at my local dealer.