I don’t know about others, but I got introduced to the Mill when I responded to one of the guy’s ad on ARMSLIST.COM (only good for local FTF transfer, but use to be a good source of what was available in state). It was for a 10mm P220 I was looking for. Once you get introduced to one of the guys (and girls), you’ll get to know who deals in what. Most of them have their own special areas, but some of them are more like your local gun shop. I’ve done business with Dean Safety, Newtech R&D, Custom Defense Solutions, Minuteman Armament and a few others.
Just look at the NES Dealer classifieds and you'll see ads for some of them. If you see something you like just PM them and setup a meeting. Once you get to know the place it’s a real kick.
Many of the FFLs are there primarily to support their own firearms habit. I looked into it myself and took one of Jim’s “How to get an FFL License” classes. My main goal was to be able to own suppressors in MA, which really requires you to have an 07 FFL. Turns out the cost and time involved wasn’t really worth it to me (mostly because I live a bit too far away from the Mill to make it there with any regularity). But other people find it worthwhile, particular if you’re into MG’s. Since you do need to “be in the business”, all the FFLs in the Mill do sell firearms, though some only do it out of state via the internet, shipping only to out of state FFLs. Though others have full shops, with inventory and displays, just like any old LGS.