I’ve had some from an ebay shop International Bike Disc Brake Pads (sellers name is “bikefridge”. The plain non-wavey round ones have quite a large braking surface, so better suited to brakes with larger pads – I’m using them with old Hope Minis (180 front, 160 rear) and they’re fine. I’ve got some old Hope M4’s on another bike and they’re not so good on that as the pads are smaller so they only make contact with a comparatively small area of the brake disc (though with two pads, they’re in contact with a longer section of the disc, if that makes any sense).
G