Numberplates aren't just numberplates - there are subtle variences and styles according to age.
Put the wrong one on and it's blindingly obvious.  The number of times I've seen proud US owners with a yellow plate on the front is worrying...

I get mine from here - https://www.tippersvintageplates.co.uk/ and they have a decent selection of everything. 
Not the cheapest, but you absolutely get what you pay for.

Hugh, thanks for the triple C recommend. I've ordered from Philip in the past. I just ordered the British Tags with him now. It was too much of a hassel with the British companies as they want the V5#s and proof of ownership.  Philip says hello. J

Hi Jim, here is another, maybe what you are looking for.

Regards, M.

https://hillsnumberplates.com/products/finished-plates/

Try these guys, a bit spendy though.

https://www.jdmplates.co.uk/

Hi Jim, coincidentally, a few days ago I saw that some guy in the UK was making them to whatever license plate number you wanted, even going back to the old B & W ones. I'll do some digging to see if I kept a link.

Regards, Martin.

Does anyone know where I can buy some Reflective British or European Style License Plates? The long type. Roughly 4.5 high by 20.5 inches long. Prefer a black border, does not have to embossed. I prefer reflective plates.  I'm looking on eBay at the moment. Thanks Jim