Kildare Animal Foundation have an annual fund raising Open Day in June or July and there is a gentleman there each year who sells plants to help out the charity. I have seen people give him their old pots and given him some myself. He is always appreciative of them. Perhaps you could check with the rescue to see if they could hold them or pass them on to him in due course. It is a great day out and a fabulous charity so well worth the effort to maybe even make a visit yourself