What should I do with the donated amount that was designated for purchasing a specific item, but it later became impossible to buy the item and it was obtained from another source, knowing that the amount was spent due to my need for it and now it cannot be returned to the donors, and the donation was personal to me but for the purpose of purchasing this item?

The fundamental principle is that donated funds should be spent on what they were specifically allocated for. If this becomes impossible, it is not permissible to spend them on anything else unless the donors agree or the funds are returned to them. This is unless it is customary or the situation dictates that the donors would not object to their donations being spent on other needs of the beneficiary.