Hello and welcome to the site!

Have you discovered the Google Books search engine feature?
I have amassed quite a library of Victorian era books, journals, and other works via searches through that wonderful tool.

On the 12 bore (12b) rifle, there's a handful of light 12b rifles for sale on the Gunsinternational.com site

An A. Henry
A J. Purdey
and
A Greener

It doesn't appear as if any of these three are currently running *supplied with good regulating loads, ammunition, loading supplies, or tools* but with a fair bit of work and resourcefulness you could likely get any of these up and shooting without too much hassle.

At least the images on the linked site will help give you a sense of what a proper 12b SxS double rifle from the Golden Age looks like.

There are examples detailed and discussed here on NitroExpress too.
Here's an very nice example of a J&W Tolley 12b rifle, much heavier than the three shown above, also with some very cool features and engraving.

Use the search engine here on the site, get a sense of the vernacular, and go out on the Google Books search utility and find proper period writing on the subject. You'll definitely have fun reading!

--Tinker