The usage of the 8.8 in ground support role predates the Second World War. During the Spanish Civil War the potency of the 8.8 while attacking ground targets was already known. The first major use against tanks was likely the Battle of Arras.

Even 8.8 units which were pure anti aircraft units would likely have some anti tank rounds stored. I guess that excludes batteries which were stationed in Germany for the "Defense of the Reich" but all batteries in near to the front got some rounds for ground targets.

Some interesting snippet in regards to the 8.8 and its anti tank role.

"Hitler's own technical grasp was a constant source of astonishment to his advisers... ...and among other things it was reputedly he who first suggested ( in 1938 ) that the 88-mm, anti aircraft gun be uses in an anti tank role..."

To lose a Battle Alistair Horne.

Not sure if its true...

Just wanted to point out that there were in fact many "88s" in German service. 88mm was a caliber they've been using for a while, like 37mm, 50mm and 75mm. Not every 88mm was the same gun, at all. There are Flak variants, AT variants, tank variants...