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.
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).