Need help finding a 3/8 to 1/2 inch connector for water inlet valve on toilet to water supply.?

Water was leaking from my toilet. Found it was a cracked connector, so searched for another on internet and every plumbing store and can't find one. Please help, The connector is for a water inlet valve which has a 3/8 inch thread, this screws into the connector. The connector then expands to a 1/2 inch tail which a braided water hose screws onto. I tried to make the repair using a simple brass 3/8 to 15mm hex bush, but the head of the bush is too small meaning that there is not enough contact area between the hex and the cistern and the seal between the hex and the cistern is only fractions of a mm smaller than the hex head, so not enough contact between the hex and seal either! I'm hoping someone knows of a 3/8 To 1/2 inch connector. The water inlet valve is bottom fed and made by the firm Roca.