I think guys like Biyombo and Perk would be the easiest to acquire without giving up much of anything.
Milwaukee should be willing to give up one of Henson/Sanders - seems as though they are leaning towards keep Sanders. Same thing with Minny - Pek or Dieng should be available. In this grouping, you probably need to give up something of value, but nothing crazy (1st, expiring, bench guy).
I obviously would love to get Horford or Gobert, but the price is probably sky-high. I also think Utah is re-considering Kanter as trade bait.
Hawkeye brought up a good option in Lopez. BKN was supposedly looking to deal him this summer - obviously he is injury prone, but can be a legitimate 2nd star when healthy. He is still so young and extremely productive offensively. It's too bad we didn't grab his brother when we had the chance - he is all defense.