We had a few issues finding proper flashing for our new steel roof until someone pointed us to https://www.metrotile.co.uk/accessories/. They’ve got all the roof accessories in one place — from cover flashings and barge boards to vent systems. I ordered through my roofer, and everything arrived within a few days. You can tell the materials are solid; even the nails and screws feel stronger than what you’d get from a standard hardware store.