Most Yidams are given through initiation. If we have a lineage that selflessly seeks the happiness of the world, all reasonable and serious seekers are given Yidams and connected to guides. If we have lineages vain of power and tradition, relatively few are given guides. When we have egotistical lineages that hide teachings, then it is rare to find followers with Yidams, and they must wait many years for the lama (or teacher) to decide that they are worthy. This is the lowest type of lineage.
It is not up to the lama or acharya to decide the worthiness of a candidate - it is up to the dakini or bhairava. If these guides do not like the person, they will not come. But they decide the worthiness, not the lama. For the lama to claim that role is to exceed his authority.
As a general rule, if a novice of good moral character cannot receive a guide within a year, then the lineage is taking undue power, and the novice must look elsewhere. Very little transcendent insight comes from external teaching - all true transformational knowledge comes inwardly, and should be guided by a dakini or bhairava.
If a lineage clutches at its Yidams “like buried treasure”, then the novice must go elsewhere. He or she usually has three choices. One is to go on pilgrimage to sacred sites where monks and saints have done meditation. Sometimes a link is maintained at such sites to the deities involved. Prolonged meditation in such places may rekindle the spark, and the past visionary atmosphere may return (so the seeker may contact a Yidam there).
The second choice is meditation, which creates a link. Extended meditation and concentration on finding a Yidam can slowly create a brilliant meditative flame around a person, and sooner or later, the sheer intensity of desire and concentration will capture the attention of the bodhisattva, bhairava, or dakini. It is the sheer desire for a guide which will draw one down if the person shines with the light of effort.
The third method to contact a Yidam is through dream yoga. There are some bhairavas who prefer to work with people who are fully conscious. However, there are also guides who prefer to work with the inner bodies while the person is asleep. These are specialists and may be called down in different ways.
One technique is to visualize the inside of the physical body as perfectly empty and still. It should appear as dark with perhaps only a few stars. The only winds are those of peace. Drops of nectar should be visualized as falling on invisible waters which respond with concentric circles of blue light. In the center of one of the circles is the bija (seed) mantra HUM, lit up in radiant blue light. This cleanses the inner body which then sparkles with subtle blue light. As the novice rests, he or she should chant the mantra OM BHAIRAVA HUM. By doing this each night, the mind is cleansed and a pathway is developed for the entrance of the guide.
It is important to note that guides come at the beginning of a journey, not at the end. To put off a guide until years of philosophical training are over, kills spiritual development. The guide is most vital at the beginning, to untangle universal truth from lineage jargon.
If you seek a Yidam, seek a beautiful one to begin. One’s path should begin in a positive way. Yidams can always take on their wrathful forms but it is no way to begin. Destruction emerges when it is necessary, but stepping onto the path is a creative choice. Visualize the light that reflects your inner being best, and surround yourself with it like a great flame. Call upon a specific deity, or any bodhisattva who harmonizes with your light (or personality). You must see with your heart that you do not have a ghost or demon that responds to your call but a true guide. Ask his or her name, visualize the name and see if the being stays. If it stays true and bright, ask if it will guide you on the path. (If he or she says yes, then) this is your Yidam.
Gaining and communicating with a Yidam is certainly an esoteric practice, and one on which some seekers spend a great deal of time and effort. Though the above mentioned difficulties associated with certain spiritual lineages are sometimes the reason that students are not given Yidams, there is also another very good reason why seekers are not assigned Yidams:
To gain a Yidam, people in lineages must request one. Then the teacher must gauge if assigning one will do any good, as the major Yidam problem is that people cannot see them. Even if they have one assigned, it does no good if there is a wall of ignorance between them (the seeker and the Yidam). The Yidam could be there but the person cannot see or hear them. It is a waste of time on both sides.
Yidams can protect the seeker from distractions and demons, motivate and guide him or her, and explain the paths of spiritual development.