Well, yeah that's the question. These days, your surplus store is on ebay. And being surplus you have to grab it when you see it. So the prices are there but the availability is wait until it shows up. The other source is still gun shows. I see lots of good surplus in my area, but you may have to travel to find it. The surplus stores are sadly no longer reliable sources.
You can find legit kit on ebay, you just have to be selective in your vendors. The good ones only sell the real stuff, and have good reps. You may have to sort through a bunch of crap, but it is there. Maybe not when you want it, but if you keep coming back, eventually you will (probably) find it. I buy lots of kit from there.
The gunshows in my areas still have a few guys that have genuine surplus at decent prices. But you have to hit the bigger shows to find them. Which is a pain in the ass, but doable. You just have to hit the times where stuff is surplused and available. So it becomes a constant monitoring process. You just have to be ready to buy whenever stuff shows up.
I try and hit the surplus wave when new stuff is being cut in. For example, OCP is being surplused out as big army gets ready for "scorpion". So you stock up on the OCP stuff. I found the cammies, soft/hard shells, windshirts, etc, at gun shows. Also the packs and LBE. I found LBT 8-pocket rucks on ebay. I found surplus OCP materials at a fabric wholesaler.
And then there are the new finds, like Crossfire out there. So yeah, we're on the ground floor of their ruck systems coming out. You can go ahead and get the frame and suspension now, and throw your own bag on it, and then upgrade to their bag when it's available in OCP. I think these guys are going to be a great source of not only complete ruck systems, but also other LBE.
So yeah you have this mix of new cutting edge, and the older surplus. Kinda a balancing act there. It just takes constant work to ferret it out, no quick solutions as a rule of thumb.