DIY Photography has a very good article by Corrie White called The Comprehensive Water Drop Photography Guide. Corrie is one of the leading drop photographers.
Corrie White produced a book on drop photography. I don’t have it but there are many good comments on line. See Corries website at https://www.liquiddropart.com/
Wayne Fulton’s scantips website has some very good information on valves, nozzles and drops.
Tobias Braeuning has a good article about his set up including the circuit diagram for the device he built (in German).
Kevin Lewis’s PhotosbyKev website has a another good guide (read the comments) called Water Droplet Photography.
strobist: How to Photograph Water Drops with One Speedlight
Video Tutorial by Daniel Nimmervoll (in German) Wassertropfen Highspeed Fotografie Folge 1
Daniel has several videos on youtube that clearly show the 2 drop method for getting taller sprouts. Start with this one. and then see this one and then explore is whole channel.
Advanced 3 valve Setup by Daniel Nimmervoll (in German) Mein Aufbau für Wassertropfen Fotos
Drop Art Photography Facebook group. Group dedicated to water drop photography (not my group).
GarethB has an excellent write up (really detailed) on the talkPhotography forum called An Idiots guide: DIY Water Drop Controller with Arduinos and stuff. Gareth goes in to great depth about building a controller.
PetaPixel has an article written by Ted Kinsman called How to Build a DIY Double Water Drip System for High Speed Photos
Bryan Rolfe on Instructables has an article showing how to perform High-Speed Droplet Photography With an Arduino
Other DIY Devices
Ramigrafx has another Arduino based water drop controller with full build instructions.
Photoduino. A multi function camera trigger.
Michael Ross has a comprehensive guide for building a High Speed Photo Trigger based on the Arduino Mega
Here are some of the commercial products you can buy. Most of these are expensive which is why I built my own.
HiViz AstroSplash Drop Controller
MJKZZ water drop photography controller
The Drip Kit
Splash Art Kit
The Time Machine
DIY Shutter Release Cables