Funny you should bring that up. A couple of years ago, I became interested in the origin of tulpas and did a literature search (both on and off-line), talked to a friend of mine who is a Dzogchen practitioner and to a Tibetanologist (Geoffrey Samuel, who's book "Civilised Shamans" I'd reccomend, btw).
Virtually all the tales about Tulpas comes from two sources - either Alexandra David-Neel's accounts of her fantastic journeys in Tibet in the 1920s or from Evans-Wentz's "The Tibetan Book of the Great Liberation":
...'mediums' in the Occident can, while entranced, automatically and unconsciously create materializations which are much less palpable than the consciously produced Tul-pas [thought-forms], by exuding 'ectoplasm' from their own bodies. Similarly, as is suggested by instances of phantasms of the living reported by psychic research, a thought-form may be made to emanate from one human mind and be hallucinatorily perceived by another, although possessed of little or no palpableness."
Both Evans-Wentz and David-Neel were Theosophists, so I cannot help but wonder how far their accounts of Tulpas were influenced by the writings of Besant, Leadbeater, et al. Certainly, most Tibetanologists nowadays agree that Evans-Wentz's books owe more to his Theosophical leanings than to any great familiarity with Tibetan Buddhist material.
Also, there is some contention of the term itself. As far as I was able to discover, Tulpa is an anglicised spelling of sprul pa, which has a number of meanings in Tibetan, such as 'body', 'prescence', 'projection' or 'incarnation' or "emanational body" - which sounds as if it could refer to a projected thought-form, but is probably more likely to be a reference to the Buddhist Trikaya doctrine wherein the emanation body is the body (usually of a bodhisattva or the historical Buddha) which appears to the human senses. Indeed, this is how Mme Blavatsky uses the term in the earliest english reference to Tulpa I found, which is in Vol.3 of the Secret Doctrine.
In any case, apart from David-Neel's direct accounts, I have not been able to find any references to Tulpas in terms of primary/secondary sources, apart from some vague weebling about "Bon Shamanism" and so forth. It's generally assumed that tulpas are the same as servitors, and David-Neel's accounts are occasionally wheeled in to back up the assertion that if servitors are not carefully managed, they will "go out of control". In over 15 years of creating thought-forms & servitors, I can only say that I've never experienced this to be the case.
this article, which reviews the Western encounter with "Secret Tibet" may be of relevance here.