I'll run around HP and see if I can spot these herbs. Not sure why I'm not seeing any of them yet.
It's mainly because there aren't as many nodes in Outland and Northrend as there are in the pre-60 old world; they added lots of extra herb and ore nodes when they revamped Azeroth with Cata, so that's why you've been tripping over things up to this point.
Felweed is a big green plant, and you should find more of it in the western half of HP, and less of the dreamfoil and mountain silversage. You'll see a lot of both in Zangarmarsh as well, and frankly I prefer to gather in HP, then hop over to Terokkar when the mobs aren't as dangerous, avoid Zang altogether. Also, nightmare vine from Shadowmoon Valley might sell well in the AH, since it's used for the final wizard oil that casters can apply to their weapons.
From now on, you don't have to mill such specific herbs to get specific pigments, there'll just be the regular and the rare pigments from any herb from an expansion's zones (Outland, Northrend, Cata zones, Pandaria). When you get to Northrend, Sholazar is loaded with herbs, but I've found icethorn and lichbloom tend to drop Icy Pigment more often, which is needed for Northrend Inscription Research, and those are common throughout Storm Peaks (although doing the Sons of Hodir quests makes it MUCH easier to gather).
This may help too: