These suppliers are amongst those I have used recently; most are based in the UK. Other suppliers are available!
Lucy Coad: Tuning hammers, plain and covered strings, Seccotine, leather for hammer-hinges, vellum for hopper-hinges.
David Law: Brassware including hinges and Broadwood dampers, tuning hammers.
Graham Walker: authentic woollen green, red and white cloth for keyboards, hitchrails (Broadwood) and dampers.
Malcolm Rose: Red brass, yellow brass, and iron wire for strings; copper, tinned copper, annealed brass and soft iron for covers.
Optimum Brasses: Brassware including casters, bolt-covers, and Broadwood dampers.
Sykes Timber: Timber including walnut, spruce, pine, poplar, beech, oak, cherry, etc.
Elforyn: High-quality synthetic ivory replacement.
Casein: Natural ivory replacement
The Music Room (Andy Durand) Covered strings, leg-threads