I made this Riverbud farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. You can pick Riverbud with BfA Herbalism skill 1, but the rank 2 and rank 3 requires higher Herbalism skill. Higher ranks allow you to gather this herb more effectively, so you will get more herbs overall.
Riverbud only grows near water like rivers, small lakes, ponds. But you won't find it in salt water near the sea.
Riverbud
Riverbud ranks
You will learn rank 1 from your Herbalism trainer at Daz'Alor / Boralus.
Rank 2 is a reward from 
Emergency Transplants. This quest unlocks at BfA Herbalism 50 and is given to you by your Herbalism trainer at Daz'Alor / Boralus.
Ranks 3 is a reward from the quest Breaking The Food Chain. This quest unlocks at BfA Herbalism 145 and is given to you by your Herbalism trainer at Daz'Alor / Boralus.
How to speed up farming
Battle for Azeroth introduces three new items that you can use to speed up farming herbs. You should use these if you can find them cheap at the Auction House, or craft it yourself if you can.
[Monel-Hardened Stirrups] - You can gather while mounted.
[Monel-Hardened Hoofplates] - Increase mount speed by 20%.
[Coarse Leather Barding] - Prevents you from being dazed. (mobs won't dismount you)
The one I would recommend the most is [Monel-Hardened Stirrups], which lets you gather herbs while mounted. Every mob scales to your level in Battle for Azeroth, so it will be hard to avoid the mobs attacking you while you try to farm herbs. If you have the 
[Sky Golem] mount, you can use that instead of the Monel-Hardened Stirrups
If your class has a tanking spec, you should switch to that while farming, so you won't get dazed and you don't have to use Coarse Leather Barding.
Don't forget to enchant your gloves with 
[Enchant Gloves - Zandalari Herbalism] or [Enchant Gloves - Kul Tiran Herbalism]! This enchant brings gathering time down from 1.5 seconds to 0.5 seconds. This is really important if you gather while mounted because mobs can still interrupt your casting.
Tiragarde Sound
This is the best place to farm Riverbud. You will also get a lot of Sea Stalk, and some Siren's Pollen.
Stormsong Valley
You will get a lot of Riverbud and Sea Stalk here. Really good route, since you can avoid fighting most mobs.
Zuldazar
This route goes through a few horde cities so it's mostly recommended for Horde players.
Nazmir
You will also get a lot of Anchor Weed following this route.
Drustvar
You will only get a few Riverbud here, but it's a good herb farming route overall. You will get a lot of Winter's Kiss and Siren's Pollen. Following the yellow route is enough to allow the herbs to respawn from where you started, but you can take a bigger loop around the zone. (marked with the red line on the map)
Tiragarde Sound
This is a good route if you just want a little bit from everything. You will get mostly Sea Stalk and a few dozen from all other herbs. (no Akunda's Bite)
