Most commonly uchen
is used in pecha, at least pecha which have been printed from xylographs, and in modern printed Tibetan materials. A nice Unicode uchen is Jomolhari, which can be found here: http://www.library.gov.bt/IT/fonts.html
The Dorje Sempa a.k.a. Vajrasattva mantra, like all mantra, is in Sanskrit, so in order to type it out you'll need to make use of some of the special characters and stacks which Tibetan uses for Sanskrit sounds which do not occur in Tibetan. If you don't know Tibetan, and haven't yet had the pleasure of experimenting with the multitude of mostly functional input methods, you're in for a treat.