www.wildcatband.com/keyed.html - A Pennsylvania (USA)-based Brass Band using keyed and early valved instruments performing 1830's repetoire in authentic dress.
Andre Bissonet sells it all. He speaks a little English but call ahead to assure a range of instruments to choose from. There is no web site unfortunately: Instruments Musicaux Anciens, 6, rue du Pas de la Mule~75003 Paris, Tel/fax: 01 48 87 20 15
(see Gallery for photo)
Try www.ebay.com as a few turn up now and then. Try doing it on a national level, for example. www.ebay.fr for France.
If your ever in Lyon drop into Bernard Boch/Richard Pick or B&P Instruments de Musique Anciens Et Modernes at 8, Quai Romaine Rollande for their huge display of Muller of Lyon brass instruments. They have about 8 Ophicleides on display with the occasional one for sale. Online at www.pick-et-boch.com
A great place to dream is www.renard-music.com always has several instruments available at good prices.