I bought a SP this past Friday. My LGS had 6 Sp101 models (mix of .357 Mags ; 2.25", DAO and 4" .22LRs) and 4 GP100s (all 6", they sold out the 4" models they had last week) in the display case. When I asked if he had any in that back that were not handled by the public, he said he did and came back with an untouched one. Only store in the area that had them though.
I had given up, but stopping at every gun store in the region scrounging reloading supplies, I found a 4" GP-100. The guy had 3 others in the case. In my travels around Ohio, I saw a fair number of Smiths, Taurus, Ruger Blackhawks, and a couple of the 4" SP-101 .22s. No other double action Ruger revolvers.
AFAIK, Ruger makes guns in batches, with the more popular (best-selling) models getting 2-3 production runs per year; while less popular models may only get run once every-other year.
When it rains, it pours; when it doesn't, there's a drought.
IDK what Ruger's going to be producing in their new SC plant.
I bought my single ten and a used GP100 4" in the past two months. My most frequented LGS has had new and used GP100 6" ers regularly, new SP101 in .22 and I think they had a .357 for a little, and LCRs in various flavors. Patience and persistence.
I'm not big on Gunbroker.com . I suggest www.gunsamerica.com or www.slickguns.com .
I've seen a few great deals on DA Ruger revolvers. I like the Wiley Clapp limited ed GP100 .357magnums. They are now in target grey or stainless.