Is it permissible to work as a delivery representative for computer supplies to entities that might use them for unlawful purposes, such as banks, and what should we do if most available jobs are not entirely free from unlawful elements?

Merely anticipating a request to deliver items to a usurious bank does not preclude contracting with a company for a permissible job. If you are asked to deliver devices used in the operations of a usurious bank, you must refuse, because assisting in usury is not permissible. If you have contracted and they oblige you to do such work, it is not permissible to continue in it unless you are in a state of necessity, such that you cannot find your necessities and those of your dependents except through it. In such a case, you may continue working until you find another job.