One of facts of life if you are interested in Golden Dawn today is that most people are either self-initiates, or long distance members. Very few people are members of an actual lodge. Most people experience the system through self-initiation or "astral initiation" rather than actually undergoing the rituals in a Temple setting.
There is a whole can of worms involved with both self-initiation and astral; you can boil the worms down to that sticky, dark goop of a question "Do these variations work?" Being a little fuzzy around the ears today, I chose to delay addressing that question.
Instead I want you to consider how long the concept of self-initiation has been around. When I became interested in ceremonial magic in 1991, one of my text books was Modern Magick: Eleven Lessons in the High Magickal Arts by Donald Michael Kraig. This book came out in 1988, and has been in print ever since, much to the surprise of its author.
When I encountered Andew W., the man who was to become my sponsor and mentor in Hathoor Temple, I was reading this book. In fact, it was how Andrew spotted me. Upon leafing through the book, he laughed. At the time, I was under the impression that he had a low opinion of all who studied such things--later I realized that he had a low opinion of the book as did most of the members of Hathoor Temple.
In hindsight, with fifteen years of Golden Dawn experience under my belt (most of it Temple based), I have to agree that perhaps Modern Magick is not the most ideal book for the purposes of self-initiation.
But it is not the first book on self-initiation, nor is it the last.
In 1995, Chic and Tabatha Cicero had their opus Self-Initiation into the Golden Dawn Tradition: A Complete Curriculum of Study for Both the Solitary Magician and the Working Magical Group published. I know quite a few people who have used this book. As a Temple Initiate, I must say it hardly prepares one for the curriculum of the SubGrades of Inner Order (ZAM, THAM, etc.) or at least not the program that I have the most experience in; that particular Inner Order curriculum depends upon knowledge of the Temple versions of the rituals.
The Ciceros will not be the last to publish such a work. Why? Because with the inability of individuals to find local Temples, and the problems with long distance memberships, self-initiation fulfills a need. As long as the need is around, self-initiation will happen.
This need has been around for a long time. The Hermetic Brotherhood of Luxor had an initiation script "The Hermetic Ritual For the Private Initiation of Neophytes" which was to allow for their members to self-initiate. Most of their members were self-initiates, especially after their system of lodges collapsed.
The A.A., the famous (or infamous) organization detailed in Crowley's Equinoxes is primarily a loose band of self-initiates.
In fact, advancing a member without them undergoing a Temple ritual has been so common throughout the history of the Orders, there is a Freemasonry term for it--communicating the degree. Basically, you sign your oath, and are issued the relevant papers for your new degree.
How far does the concept of self-initiation go?
My personal opinion is that it extends all the way back to the writings of Agrippa. His is the first book that I can look at and see a set of lessons for the self-initiate in. Whether that was his intention or not is hard to say.
Self-initiation is going to be around for awhile. Besides the sources already mentioned, there are some non-Golden Dawn systems of self-initiation--the works of Mouni Sadhu, Edred Thorsson, and Franz Bardon are all designed on the principle of self-initiation. For those who are more interested in Golden Dawn with a side order of Thelema, check out the webpage of the Thelemic Golden Dawn. And for ceremonial magic, there is also the works of John Michael Greer. Wicca has Scott Cunningham as a source.
And there are probably many works and options that I do not recall, or have not read.
Bottom line, if you are interested in doing the work and can not find a suitable Order, there is self-initiation.