Is it permissible, in Islamic law, to train (without pay) in banks on technical matters, and is it obligatory to search for an alternative source for training?

It is not permissible to work or train in usurious banks, because the one who works in them directly engages in usury or assists in it. They are a place for a great evil in which major sins are committed, and a curse may befall one because of it. Furthermore, training in them may lead to continuing to work there due to temptations.