We have normally made our own in the past, if you are interested let me have an email address for you and I'll send over the cct diagram.
The last one we acquired ages ago came from GU Medical now Fannin UK? It might be worth giving them a call to see if they still do it - I think they are based in Wellingborugh somewhere. If not try contacting GU medical in Reading as through my serch engine they come back as a supplier of Ac fibrillators. Otherwise it is the good old US of A - try a serch for ac fibrillator suppliers in USA, or start with Cardiovascular Instrument Corporation.