where do the Buddhits get their information about the "bardos"?

Answer 1

The word bardo literally means "an interval between two things." Bar means 'interval' and do means 'two'. We can think of this interval in a spatial or temporal way. If there are two houses, the space between them is a bardo. The period between sunrise and sunset, the interval of daylight, is a bardo. A bardo can be of long or short duration, of wide or narrow expanse.
To a large extent our experience is made up of intervals between one thing and another. Even in the case of the momentary thoughts that arise in our mind, there is an interval between one thought arising and fading and the next thought appearing. Such a gap, even if infinitesimal, is part of every process. Everything we experience has this quality of intervals between states.

Answer 2

From the Tibetan "Book of the Dead".