[MarkSheppard]

In the early months of Barbarossa, the Soviets weren't in any position to hold onto the battlefield to recover such things like abandoned tanks, or tanks with minor damage.

By August 1941, the front lines had stabilized long enough and the Soviets had launched a few successful counterattacks, etc.

I'd say that from September 1941 to March 1943, captured tanks can only be bought singly, and for a very high cost; to represent individual tanks being captured, and turned against their users at the battlefield.

From March 1943, captured German equipment becomes cheap and commonplace; and you can buy platoons; because by this point, the Soviets have captured hundreds of Panzer IIIs and other vehicles in the encirclement at Stalingrad, and have had time to repair them.

From that point on, the Soviets would almost always be in command of the battlefield from Kursk onwards, so German equipment is available in quantity.