I'm pretty big about Hugh Wood Insurance of London, Toronto, and New York.
They specialize in collectibles, both collectors and dealers, and are the insurers for the American Philatelic Society, including the circuit books, which says a lot.
They don't offer one -off shipment insurance, but their annual contract covers fire and water damage, transit (both shipments and to shows -when those start again), consignments ,and shoptheft, as well as being capable of understanding that the collection/stock can vary in value from day to day. Their rates are very reasonable compared to generalist commercial insurers.
