I couldn't find any using Google searches either. It seems like it would be a valuable type of drill.
Drills to practice tempo gains?

You ask an important question. If there are drills or exercises for tempo gains, I have never come across them. The closest would be certain endgame studied / positions, where one tempo is often the difference between winning and drawing. That doesn't sound like what you are after, though.
Most opening principles (don't move the same piece twice, no unnecessary pawn moves) are based around tempo conservation. Studying them might help. Take the following (admittedly slightly ridiculous example):
The best discussion I have seen on tempo is in Tarrasch's The Game of Chess. It is in the opening section, and though he is dogmatic and mathematical and should be taken with a grain of salt, it is excellent and still very valuable. You can find a copy quite cheap nowadays, though it will be in the older notation.
Edit: I created a YouTube video for someone else, but I talk about tempi throughout, and specifically at approximately the 20min mark.
Hello everyone,
I am reading "My System" by Aron Nimzowitsch and have spent a week studying the first chapter on "The centre and development". I am fascinated by this concept of tempo and how winning tempi can lead to a dominant position. Are there any drills or puzzles I can access that focus solely on tempo? I feel it would help me with position but also with thinking ahead three to five moves. Like in the book Nimzowitsch shows an example of moving Bd7, which seemed ok. But four moves later white was three tempi ahead and black had nothing in the center with all material equal. It was like magic. I feel I need drills specifically for that.