The particular entry concerned a range of decorated T-shirts, and comes from America. The website is https://topkency.com and they produce, among other items, T-shirts styled as military uniforms, varying from medieval to modern. The picture below is of their take on Nelson. As they are priced in US dollars I'm not sure how they cost out in the UK, but that can be discovered easily enough.
So now you can dress in the style of your game!, enjoy your browse of the site.