Why? We gave you the answers you were looking for. Every male 18-49 is in the unorganized militia, they can be called up for service at any time. The closest thing to what you are looking for is the NY Guard. Their is no big underground army that I am aware of.
If you can find one it's probably one of two things.
1. A bunch of crazies who think Germany is just biding their time to launch a full schale invasion of the US to finally get their revenge for WWI and WWII, the aliens are coming, or some other wired story.
2. A group of friends that shoots together and they jokingly call themselves a militia but are really far from it. This would be closer to a gun club then an organized militia as they just practice shooting and or hunting. You would be hard pressed to find a gun club that practices military drill and such.