most local motor factors will do them too.
I don't know how "authentic" they are though, in terms of classic cars. I don't remember too many pre '87 cars going around with pressed black'n'silver plates. I remember the raised plastic digits on red/white plates or "snazzy" black plastic ones with silver digits milled into the back of the plates.
Mind you, I was only a toddler in '87 so some of the older folks might remember better.