I’ve always had a “www.MINI2.com” banner on the back of my MINI; back in 2003 I was able to buy one from MINI2, and when I upgraded to GBMINI#3 last year, someone in England was making duplicates.
Now though, for GPMINI, there’s not much choice – so I followed Josh’ advice and had my own made by SpeedySigns!
It’s a little taller than the original but it’s fine (cost, about $10 per decal – plus shipping).
Speedysigns requires a minimum decal height of 2″ so I used the extra to make a couple of website links – one for each car (also shown above).
M&I / GBMINI 
They look good. Mine weren’t laser-cut like that (or did you cut around the letters yourself?)
Josh, that’s how they came – the little ones had to be backed as you see, but the big letters were apparently big enough to be individually cut out – maybe it’s because I did it too big!
The original mini2 ones were all individual letters spaced onto a backing sheet. I wonder if any sign places can do it like that…