Is there any good source/textbook with notes about db indexing with examples?
The wikipedia entry is informative.
Database Systems: The Complete Book by Hector Garcia-Molina, Jeffrey D. Ullman, and Jennifer Widom is probably good. Older versions of database books by Ullman have been accurate and reliable. I haven't seen the latest version, though.
Actually, most of my experience with Ullman has been his compiler books, not database. Still, though, a reliable person. More than just a random author.
The text book I use for teaching database indexing is "Database Systems Concepts" by Silberschatz, Korth and Sudarshan. The book is regularly updated to keep pace with the multiple advances happening in the data management domain.
The chapter on indexing in the book is good and has a few examples. You can get a feel for the examples from the slides made available by the authors at https://www.db-book.com/db7/slides-dir/index.html
For B+tree indexing, you can find many visualisation tools online. I have used the below one on many occasions: https://www.cs.usfca.edu/~galles/visualization/BPlusTree.html