Shogen (literally: Source Document) are basically large compilations of different ways to represent the same ideogram, usually assembled by reproducing famous calligraphers' works, or by copying engravings from temples and sculptures.
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From what I gathered so far, getting your hands on one of these volumes is not exactly easy.
The one I finally got, for example, went out of print around 1999, and finding one is not easy even if you are proficient in Japanese (which I am not).

Your best bet is to either visit a second-hand bookshop in Japan, or see if you can ask a local friend/contact to do that for you.

I tried a few of the usual ways to get a book on Inernet (e.g. abebooks) but to no avail.

In the end my own teacher kindly offered to get it for me (she has relatives in Japan who located a few copies allowing me to choose which one I wanted).

The receipt accompanying the book refers to Tengu Bookshop. Here is the basic info I managed to piece together for my copy:

Title: 書源: 名跡六体大字典
Shogen: Meiseki Roktai Daijiten [Complete Dictionary of Calligraphy: Famous Examples in Six Types of Writing]
Author: 藤原鶴来
Publisher: 二玄社 (Nigensha)
ISBN: 9784544012033 / 4544012031

Book Description: A "catalogue" of different calligraphic renditions of Japanese kanji ideograms.
This dictionary includes 4,500 characters and illustrates 47,000 famous examples in six different scripts: official script, regular script, seal script, semi-cursive script, running script and ancient script (oracle bones and bronze inscriptions).