If you're trading for a relief pitcher, target Lowe from the Mariners, or Uehara from the Red Sox. If you want control and you're willing to give up more for it, I'd call up the Phillies on Giles, the Marlins on Ramos, the Mariners on Carson Smith, the Indians on Allen or McAllister, or the Brewers on Will Smith.
Kimbrel (salary) and Chapman (assets) will both cost too much, and while Chapman is still as f***ing legendary as ever, all the other guys I mentioned have been better than Kimbrel this year.
There's an ass ton of very good relief arms that are currently pitching for bottom-of-the-barrel teams, it shouldn't be extremely difficult to find a guy or two.
Edit: Didn't know Grilli was injured.