Alex Rast wrote:

Ideally, one would want a flexible, nonreactive metal mould - thin
stainless steel or perhaps tin or brass. Generally, however, it seem
these are hard to find except as antiques. I'd look for a good
antique set - if you've found some online, I'd investigate those.

eBay has antique metal molds all the time. Anton Reiche molds
are specifically praised in Whymper for their plating, which is
nickel. The base material appears to be steel (they rust).
Unfortunately, most Reiche molds will have rust.