I think the best PG fit in the league for the C's is Dejounte Murray.
What do you have to give to get him? Smart and Bruno for Murray works. But do the Spurs do that?
. Your not serious. Maybe Smart and 4 firsts might get it done
That seems like an overpay... 
Murray is probably the sixth best PG in the league, Better if your looking at advanced stats. He's young in his prime, I would make that trade in a heartbeat for the celtics.
The Spurs wouldn't do it. He's their best player by far.