Keika hyakugiku consists of 3 volumes. As you knowVolumes 2 and 3 can be viewed on the Smithsonian Museum website.
Volume 1 can be viewed on the website of the National Diet Library in Japan, but unfortunately it is not in color.
The first edition was published in 1893, but has been republished many times since then and is available with any luck at antiquarian bookstores in Japan. This book is a well-known art book for those who like to grow chrysanthemums. Therefore, some libraries in Japan hold the reprints of this book on the shelves of gardening book rather than on the art book shelves.
I am sorry, but I hardly know about Hasegawa Keika. But i know his another collection of designs. It is Kyoka Zuan, it can be viewed on the website of the library of Ritsumeikan University in Japan.
Keika hyakugiku Hyakuz vol.1
Kyoka Zuan
In looking for a link to the book, I found another collection of designs.
It is a Kogei shinzu by Yuho Tanaka. I think it is not suitable for a traditional Japanese irezumi design but nice to tattoo design.
If you are interested, please have a look.
Kogei shinzu