Before Wrath of the Lich King, these mats were not as heavily in demand as they are now. The only use for the old enchants was for twinks, but the supply was equally low. Wrath brought BoA gear, which helped demand, as well as a lot more people farming Strat. The prices went way down as more and more people farmed that place, and the fact that the last boss dropped a mount just added to it.
There are, however, no longer any drops in Strat that disenchant into these mats. I think the best deal I got on the Large Brilliant Shards was 10 stacks at 30g per stack -- but these days, I'm lucky to find any quantity on the AH for under 20g per unit.
Supply and demand?
The demand is still there. These enchants are selling better than ever, but the farther we get into this expansion, the fewer mats are in peoples' stockpiles left over from Wrath. What this means is that eventually, once all the leftover stock is used up, the price for mats for these enchants will be considerably higher than it currently is.
Strat wasn't the only place to get these mats, but it was the most popular place. You can find things that disenchant into Large Brilliant Shards in Scholomance, Blackrock Depths, Blackrock Spire, Silithus, and Dire Maul. None of these places are nearly as farmable as Strat was, but they do represent a fair slice of content.
Generally, I suspect that unless the price for these mats gets so high that it's worth farming for, the majority of supply for the mats will come from people doing that content with a character that happens to be able to disenchant. Unfortunately, there is no cheaply craftable blue gear that you can disenchant. This will push the price for the enchant scrolls higher, which will lower the number sold. Eventually, this will balance out the reduced supply until we have a new equilibrium.
Check your bags
I discovered about 20 stacks of shards in a guild bank I use for long-term storage. I had completely forgotten about them until a good friend suggested I look around my storage, and at 400ish gold per stack now, I'm making a great profit selling them. It's well worth checking your own storage to see whether you have any of these left over.
I am also watching the auction house hoping to snag some of these for less than they end up being worth. Flipping is always a risk, but I think that unless Blizzard introduces some other source for these, the final price will be higher than I'm currently paying.
If you do decide to try your hand at flipping these mats, be careful not to get caught paying too much for them. The price will eventually settle down once the sources for mats and scroll market balance out, but there's no way to know what that will mean in terms of gold per item until it happens. A good rule of thumb is to not buy mats above the point where they would be profitable in a scroll now. It's probably safe to assume that the average scroll selling price won't go down in the long run, as inflation and scarcity of mats will help keep them up.
Maximize your profits with more advice from Gold Capped as well as the author's Call to Auction podcast. Do you have questions about selling, reselling and building your financial empire on the auction house? Basil is taking your questions at firstname.lastname@example.org.