This one. You can find them on eBay. The funny thing is I have done over 20 night sight installs over the years but the PPQ with its tappers slide design is extremely tight. Even fine needle nose plyers are a pain. The $15 tool allows consistent tightening without buggering up the nut.
http://www.opticsplanet.com/meprolig...nt-sights.html
You can purchase the Meprolight 18801 P99 compact sights at a good price here as well.
http://www.opticsplanet.com/meprolig...r-pistols.html