I was also looking to buy a new case (not sleeve) for my Surface Pro... something that would also have a shoulder strap.
...then I realized that I already had one. I just took out my old Netbook, and used my Targus 10" Netbook case, which was perfect. (it also has space to hold the charger, mouse, pen holder, and slim dvd burner).
Targus Sport 10.2? Netbook Case | TDC007US