Non-lethal tools in Dishonored, The Knife of Dunwall and The Brigmore Witches
Players coming back from Dishonored 2 often ask how to keep a Dishonored run non-lethal. These are the tools each campaign offers, as the game describes them, with what they cost; the Daud campaigns add one Corvo never gets.
Sleep darts in the crossbow and the wristbow
Corvo's Crossbow launches bolts, sleep darts
and incendiary missiles; 
Daud's Wristbow bolts,
sleep darts and explosive bolts. Both are silent. Piero's Shop sells
Corvo's three crossbow upgrades: Accuracy for 300 coins, Range for 450 and
Reload, which also enables auto-reload, for 600. Daud buys the same three for
the wristbow before each mission or at Jerome's black market shop, with Reload
at 450.
The stun mine, Daud's alone
The 
Stun Mine is "a nonlethal trap that can be placed
on nearly any surface". It exists only in The Knife of Dunwall and The
Brigmore Witches, as a 300-coin modification of the lethal
Arc Mine. An extra charge costs 600 coins, before each
mission or at Jerome's shop.
Chokes, and the charm that speeds them up
A choke from behind knocks a guard out without a tool. The Strong Arms bone charm makes choking faster, in Dishonored and in The Knife of Dunwall. Scavenger finds ammunition in greater amounts, which stretches a short supply of sleep darts.
Powers that keep guards alive
Possession controls an animal at level I (3 runes)
and a human at level II (5 runes): walk a guard away from their post.
Bend Time slows time at level I and stops it at
level II, 8 more runes for Corvo: walk past a patrol untouched.
Blink moves you from cover to cover without being seen.
The rune planner totals a build of these powers and names the mission after which you can afford it. The store prices are on the stores and upgrades page.