I usually make my own plates. Here in Aus most of the States have online systems where you can order a plate for your 1:1 car, so I just put in the plate and copy the preview image. I know there are similar options for plates from the UK. I resize the image for scale or to fit the model.
I used to print my own on decal paper, but realised I was always waiting until I had sheet full so that I did not waste the (not inexpensive) decal paper. Recently I have taken to printing the plates directly on to standard 4" × 6" photo paper. This paper is cheaper, I seal it with MicroScale Industries Micro Flat or Satin, cut out and mount with thin double-sided tape. I find the thickness of the paper is a reasonable scale match to the thickness of a real plate.
The plates on the few recent buils I posted were done this way.
Cheers,
Grant.