Found a nice one today while scouting around Whiterun;
Broken Fang Cave - Due West of WR
The area around here has lots of butterflies both blue and monarch and there are quiet a few blue mountain plants here also. Scout the area out as its wide open and the reagents are scattered around a wee bit. Managed to collect enough here for at least 20 good healing potions.
That is, pretty much, the point: you DO have to look for them and you WONT find them all nicely together in one place.
Explore, dont fast-travel and you will soon be over-burdened with things.
The further you go out into the wilds the more you will find certain things, but they are sensibly placed: swampy-things in swamps, mountian flowers on.. mountains, bees around bee-hives (and bears around bee-hives!)... bee-hives in deciduous forest (willows esp~). Barnacles, clams, fish-eggs - in the water. Fungi - in caves. Butterflies on flowers....Glow-bugs... out at night!!! ....Its all pretty logical really. ?_?
Edit:
TIP: run in third-person, zoom all the way out and look-down (in isometric view) you can now run and collect things by clicking A (xbox) without having to stop and focus on stuff, just run over things clicking "A" and you can hover them up (not too great for things that move like butterflies, but fine for flowers etc).