My first thought is "Grey Wardens", since we do know they have an interest and mounted and expedition to the thaig Hawke went to. Plus we kinda already know they're into some weird **** in DAI. There was also a behemoth in the latest trailer at the warden fortress, so it could be possible the first behemoths were grey wardens, not templars. Or it was a templar. /shrug.
From there, I don't exactly know how/why templars could have gotten their hands on the stuff. The First Warden seems like an iffy sort of guy, but I don't know if he'd be making deals with the templars. 
I could see the wardens bringing some back for study and it ending up growing like wildfire on the surface. They could easily huff the stuff, but that seems a little meh.
It struck me that Meredith knew a lot more about what was going on in Kirkwall, despite her mania, so it wouldn't surprise me if she threatened to interfere with wardens traipsing through "her realm" unless they shared something of what they discovered in the thaig, red lyrium being of key interest, given we know she got a hold of the idol sometime between act 1 and act 2, so definitely early enough to dig her fingers into finding more and slowly begin siphoning it through the ranks. Or she may have been involved with initial procurement, but it was after her demise that a Templar loyal to her/of similar beliefs started actually using the wretched stuff.
...lord, I hope they weren't using the red-lyrium-Meredith-statue as a raw source of material. Bleh.