There are plenty of people in England who are very knowledgeable about Erard and other 19th century instruments who could help you. Mike Parker and Allan Harbour are two who come to mind.
Erards come up fairly frequently on ebay. But they are more often than not worm eaten wrecks that need massive amounts of work. They typically go for around $3,000 to $5,000.
Miles – if you’re in the UK have a look at affairsoftheharp.com – there’s a big section of adverts and you can phone or e-mail Geraldine, who is very helpful (and charmingly eccentric).
You can contact Mike Parker through his website at http://www.parkerharps.com. Other good resources are Michael Parfett at www. michaelparfett.com and John Hoare at http://www.pilgrimharps.com. All are interesting websites in any case. Let me know if this helps!
Thanks for the very useful information! I have been in touch with a number of people and organizations now which has been an insight and a great help! A useful tip for anyone wanting history of their Errard harp is to contact the Royal College of Music (www.rcm.ac.uk) who now own the historical ledgers.