As far as I'm aware the largest source of Western aircraft to fall into Soviet-aligned hands during the Cold War was the inventory of South Vietnam. When the country fell in 1975, many aircraft fled to Thailand, but a sizeable portion of the inventory fell into North Vietnamese hands. The PAVN captured 877 RVNAF aircraft and helicopters including 73 F-5s, 113 A-37s, 36 A-1s, 40 C-119s, 36 AC-47s, 430 UH-1s and 36 CH-47s. Many of these aircraft would continue to fly with the VPAF after the war, and for a time Vietnam would be the only country in the world operating MiG-21s and F-5s in the same unit.

Of these aircraft, the F-5s are the only ones I'm aware of that would have examples provided to the Soviet Union for evaluation. It's likely that more aircraft from the Vietnamese inventory and other sources reached the Soviets as well, but I'm not aware of specific examples. Vietnam, along with the very recently Soviet-aligned Derg regime in Ethiopia, would provide examples of F-5s to the Soviets for comparative tests against MiG-21s and MiG-23s. During these tests, the F-5 performed very favorably against both the MiG-21 and MiG-23.