Yeah, nirnroot isn't the easiest to find, but if one looks around the bodies of water, a few are sure to be found. There are two that spawn regularly near the water outside of Whiterun. Traveling at night in the swamp around Solitude will also provide a few. Also, Blackreach has some. The bridges in the Reach almost always have one somewhere under or near them. Buy them when you need them, but collect them when you can.
A shame those can't be planted. I'm sure there's a mod that changes this, but I'm on a mod-free diet. I'm trying to watch my Orcish figure.