That is a good question-
Missing acft are searched for by CAP (civil air patrol) as well and those below-
Most countys have a Sheriff's posse (mostly volunteer) that does search and rescue via Jeep, Horse, Airplane and Helicopter- whatever assets the county has.
Some States have assets for search and rescue both ground and air.
State National Guard (Army and AF) have assets that can be drawn on for SAR both ground and air.
Occasionally Active Duty military bases will assist with search and rescue with ground and air assets.
Several civil volunteer organization assist with search and rescue.
There are more, but that hits the common ones.
Mark