Also, there were two Su-27's for sale privately in the US a number of years ago[2].
[1]https://en.wikipedia.org/wiki/4477th_Test_and_Evaluation_Squ...
Two Su-27s were delivered to the U.S. in 1995 from Belarus.[60][72] Two more were bought from Ukraine in 2009 by a private company, Pride Aircraft to be used for aggressor training for U.S. pilots.[73]
MiGs are not 'top secret' jets behind the iron curtain any more. Lots of friendly Air Forces have them now (India etc.) and they routinely co-operate with western forces on exercises.
The USAF seems to be pretty strict about not allowing more modern fighters to end up in private hands. There are a number of F-86s, F-104s, F-5s and I believe one F-4 Phantom still flying around the US in the hands of private collectors and air museums but nothing newer like F-14s, F-15s, F-16s or F-18s.
Edit: While I've never heard of any F-16s in Russia I'm sure the Russians have had plenty of time inspecting and probably even flying F-16s, it was a very widely exported plane.
https://en.m.wikipedia.org/wiki/International_Traffic_in_Arm...
Not publically but I would assume they've had access to Venezuela's F-16A/Bs, though those are probably in fairly poor condition by now and are nowhere representative of current avionics capabilities.
After the close of the Vietnam war the USSR, and lesserly Poland and Czechoslovakia, extensively tested US-supplied ex-VNAF ( southern ) aircraft such as the F-5E but since then such opportunities have been very limited.