Genuine antique mirrors can be found in antique shops and internet-based businesses across the UK, but there are several companies manufacturing high-quality reproduction antique frames, and some of these incorporate mirror glass with an antique look instead of standard mirror glass.
When looking at antique mirrors it is therefore worthwhile considering what type of frame and what type of mirror glass effect you want.
The advantage of going down the reproduction route is that you can essentially specify all aspects of your requirements.
For original antique mirrors, unless you are lucky enough to find exactly what you are looking for, there will inevitably be an element of compromise, though more often than not, it should be outweighed by the beauty, history or value of the mirror in question.