So sorry to hear you have lost your lamb, babysham.
It does sound as though your vet is not giving you the best of advice, which is very worrying.
I don't know about your local agri merchants, but ours produce their own versions of the branded products, so it might be worth asking them if they have a Crovect equivalent, and emphasising that you are looking for a flystrike
prevention product.
Otherwise asking around local farmers and smallholders if anyone could spare you a few doses.