Where Can I find an auto sentry in Campaign?

I am currently half way through my legendary run on the level forerunner and I cant seem to find this perk anywhere, it looks really helpful, do you guys know if there are any drop spots?

There was a bunch in the very, very beginning of the level. The first thing you should do when you start the mission is pick one up.

> There was a bunch in the very, very beginning of the level. The first thing you should do when you start the mission is pick one up.

Yep, look to the left and right as soon as you leave the portals.

Also, some nights will drop them off the field after you killed them.