Interesting question.....................originally mirrors were made by sign manufacturers like Forrest and Son or the Brilliant Sign company for clients such as large Breweries. These mirrors became very elaborate and employed many complex techniques such as acid etching, brilliant cutting, gold leafing and hand painting, they were made in low numbers, although large concerns such as Bass, probably had hundreds of large pub mirrors made over the years
The high point of this glass artform was the Victorian and Edwardian periods and fortunately there are still some great examples to be seen, in situ in some old pubs, particularly in Soho and also in museums like Beamish, which has some terrific examples.
These mirrors are very, very rare and are often very large and heavy with ornate mahogany frames and are not cheap to buy when they are (rarely) available.
Many cheaper, smaller mirrors were made to go into shops, Frys, Players and Cadburys, amongst many others, used smaller mirrors to advertise, at the point of sale, all the way through to the sixties. These are very sought after and can sometimes be seen on Ebay or at collectors fairs, particularly BBR auctions.
In the late sixties and early seventies, Victorian advertising became fashionable, first in swinging London antique shops like Trad and Dodo and reproductions stared to be made. Dodo became a manufacturer of enamel signs and many companies started screen printing reproduction advertising mirrors. They became a craze and repro mirrors could be bought everywhere, cheaply, in gift shops and for sale on market stalls. These mirrors are still made today for pubs and are largely the ones you see on EBay. They are often described as vintage and were mainly made in the 70's, some of them were quite good but can not compare to the great mirrors of an earlier age.