I know how to find them, mechanically, but thanks for the summation.
In every case, I have used ender eyes to locate the position where the portal should be and have proceeded to try to find them. In two worlds, the eyes took me to a location with no stronghold, no portal, no nothing, and no matter where I went in the world (I circled spawn at 1400-ish blocks), that same point was the only destination eyes would go to.
In many cases, I will find a stronghold, but the portal is not present. I spent a week tearing one fortress apart block by block before convincing the admin to actually bring up the world in worldedit and check. In other cases, the eye takes me somewhere but there is no stronghold, or the stronghold is, like, 2 random blocks of cracked stone sticking out of the middle of a wall, bisected by a mineshaft and a dozen ravines.
I, personally, would welcome a legitimate way to create an end portal using mods.