It has two chain loops, with a T-bar in the middle, from which hangs a medallion.

In The Snowmen it is silver; and for the episodes that follow it is a gilt colour.
It didn’t take me too long to track one down - on eBay - and here is the link to the seller I got it from. They seem to hold decent stock as it has been available for almost a year now.
Double Albert Bronze Tone Pocket Watch Fob Chain
The chain comes in a bronze metal, which for me gives a more antiqued look.
There is an option to not have it boxed, saving yourself a little money too.
The medallion is a metal shield - but this is easily changed by your local jeweller who can swap it for a suitable round medallion.
The costume from The Snowmen was on display at the 50th Anniversary Celebration, and it gave me a good chance to get a close-up look at the chain and medallion.
To my surprise it depicts a boxing match!
Does anyone know where this comes from?