3,3'-Thiodipropionic acid is listed as such in my Aldrich catalogue and is
reasonably priced, at 4UKP for 100g (other quantities available.) My copy
is a year or two out of date so the price may be wrong.
Also in the current Sigma catalogue (part of the same company)at 4.20 UKP.
Sigma-Aldrich are a worldwide company but I don't have their details for
Italy. Also known as Fluka, Supelco, Riedel-de Hahn, RBI. They are on the
web at http://www.sigma.sial.com/ though I haven't visited the site.
I think Mike Ware has published some articles related to the chrysotype in
chemical journals, but I don't have the references.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/