Couple of things.
Firstly, what do you want the pot for? Is it to make a hobo stove? I presume it's not for cooking since if it's on the outside (under) the billy then the inside of the pot will pick up all the dirt and soot on the outside of the billy. And you really don't want that, surely?
Secondly (and this is probably why there haven't been too many responses to your request) it's probably going to be a case of serendipity and luck. Good thing is, you know the diameter of your billy (12cm). so what you want to do is each time you're somewhere that may sell stainless steel homeware have a look and a measure. Those places like The Range and Wilkos are best - they're cheap and they also sell DIY stuff so you can go grab a tape measure to check the pot size to make sure! There's also ASDA and Ikea. Some have luck with charity shops. But I think you'll just have to accept that each bushcrafter finds their own where they find them rather than one killer supplier.
You'll have to keep an eye out when out and about shopping.
Best of luck.