It's pretty difficult. I tried wg-gesucht, but, as i'm sure you'll notice, they all typically require a besichtigung, which isn't all that easy, considering I'm still in the US.
I eventually found out that the studentenwerk at Stuttgart guarantees a room for one year for DAAD scholarship holders. So I'm doing that, which buys me enough time to find private housing for the second year.
This is done through Stuttgart's International Office, so I would try contacting the international office and see if they can help.