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.
Graham Walker: authentic woollen green, red and white cloth for keyboards, hitchrails (Broadwood) and dampers.
Graham Walker has been working for several years on authentic cloths for Early Keyboard Instruments, and many of us now depend on it for building new instruments as well as restorations. These cloths are woven from pure wool, of course, and are as close as possible to the original textures and colours. The range has been increased, with new options for damper-cloths. Please see Graham's website for details and prices.
Malcolm Rose: Red brass, yellow brass, and iron wire for strings; copper, tinned copper, annealed brass and soft iron for covers.
David Law: Authentic brassware (cast from originals) for harpsichords, spinets, and early pianos.
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