As far as unrestricted free agents, the two I like most for the Spurs are James Posey and Ime Udoka. Posey for a starting salary at around $3M would be nice. However, he probably gets more.
Udoka would be an absolutely perfect Bowen replacement ... except for the fact that he's 6-foot-5. Udoka can defend, rebound a little bit, decent pass, doesn't turn the ball over and is a +40% three point shooter. He's had the same type of career that Bowen had before joining the Spurs and he's the same age that Bowen was when the Spurs signed him.
Portland has said they want him back but after they drafted like 38 players, I'm not sure they have the room. If the Spurs could get him for a couple million, he'd be a pretty decent signing. His only drawback is his height, but if he were 6-foot-7 he'd cost two or three times as much.
Another thing about Udoka is he shoots most of his shots from the corner threes just like Bowen ... and Udoka hit those shots at a 42% clip this past season.