Is it permissible to use wedding supplies that were purchased with bank interest, given that I only found out about this two years later?

If these supplies were purchased by the father from purely usurious funds, then it is not permissible to use them, and they must be disposed of through charity or waqf (endowment) for charitable purposes. However, if they were purchased from money that is mixed with both lawful and unlawful elements, then it is permissible to use them, though it is disliked.