The weapons used by Cho Seung-Hui at Virginia Tech were apparently a Glock 19 9mm and a Walther P22 .22LR. The Glock has about a bazillion magazines available in all different sizes, but as far as I know the Walther has only the original 10-rounders available.
I was recently looking at the P22 myself, and I didn’t come across any larger mags. If someone knows better, pass it on.
From the Roanoke Times via a reader’s comment. Thanks!