Off the top of my head, here's a few:
Mikael Pietrus
Thabo Sefalosha
Trever Ariza
Matt Barnes
Arron Affalo
Dahntay Jones
Luc Richard Mbah A Moute
Serge Ibaka
In the cases of Pietrus, Barnes and Ariza, these guys were available via free agency at one time or another - particularly Pietrus. I know people have spoken ill of him for several years now. All I know, is that I saw him making plays on both ends of the floor for an Orlando team, that prematurely found themselves in the NBA Finals. Ariza played the same role for the Fakers, but now has way too much responsibility for the Rockets. Barnes has drifted from team-to-team for about 3 years now.
Mbah A Moute and Ibaka were both drafted by their respective teams, in the past 2 years, and were not readily available to the Spurs. However, both are superb defenders. While I doubt their current teams would be stupid enough to let them get away, they both certainly bear watching.
Of course, all of these guys have some flaws. However, as complimentary players, that are good role players and can most certainly defend. Either would've been serviceable options.