Most of the beads used by Tibetans as prayer beads - both today and in the past - have come from outside Tibet(historicaly as trade beads). So a Tibetan bead could have that loose definition as a bead that has been owned and used by a Tibetan person - but one that is more than likely manufactured outside Tibet.
Of course there is that big exception - the bodhi bead mala - which are one of the most common prayer beads. These are made of bodhi seeds - and are easy to manufacture, even in Tibet (that is if you have holy bodhi tree knocking around in the neighborhood).
So here are some pics I've taken of the real thing - and my wife trying not to buy them!!!!!
This is rural Tibet - a long way from Lhasa.