I suspect it will vary from Tradition to Tradition. The most important thing is the permission of the Guru if the Guru says you are ready then you are.
That depends on whether the Guru himself is ready; this is often indicated by approval from their lineage masters who are Vajra Masters.
After all, not all Lamas give empowerments, for good reasons.
In answer to the rest of the OP:
Yes, there are Tantric Colleges which Lamas may attend after their training (in the case of the Gelugpa) as Geshes.
Alternatively, some are trained by Vajra Masters outside of a formal college setting.
One Westerner who has been recognised as a valid master and who gives empowerments is Lama Jampa Thaye:http://www.dechen.org/dechen/jampathaye.html''Lama Jampa Thaye is a scholar and meditation master trained in the Sakya and Karma Kagyu traditions of Buddhism and authorized as a lama by his two masters, Karma Thinley Rinpoche and H.H. Sakya Trizin. Born in England in 1952, he became a student of Rinpoche at the age of 20 and met His Holiness a year later.
Karma Thinley Rinpoche bestowed upon him innumerable initiations, transmissions and instructions from the Kagyu, Sakya, Nyingma and Kadam traditions. In particular, he received from Rinpoche the famous collection of vajrayana initiations known as the One Hundred Sadhanas of Bari Lotsawa, the One Hundred Lojong Teachings, the complete cycle of Konchog Chindu and various works on Kagyu philosophy.
From H.H. Sakya Trizin, Lama Jampa has obtained many initiations and transmissions from all four sets of tantras, most notably the Thirteen Golden Dharmas, Lamdre Tsokshe, the esoteric teachings of Vajrayogini, the Jonang Transmission of the Eighty Four Mahasiddhas and the One Hundred Sadhanas of Narthang. ''