I find they are very plentiful in the beginning zones. Tons of stuff. If you do your daily alchemy writs, you will also sometimes get an alchemist's survery with map to large quantity of mats.
If you really want to find a lot here are the two things to do:
1 - Spend a skill point on Keen Eye for alchemy. Makes them WAY easier to find. Especially at night.
2 - Ok, this is the big secret. Turn off the grass. Yup, go into Settings --> Video, find grass and click it so it shows OFF. Suddenly you will see all the things. It's not nearly as attractive but it's great for farming and takes a lot of load off your GPU.