by BigShot » Feb 6th, '10, 19:03
Just a quick point.
It shouldn't be difficult to find a suitable note as there's probably no need to get an exact size match.
If you find a foreign banknote without anything too important right at the edge (better still if there's a white border) and sized slightly larger than the nearest British banknote, it should be trivial to trim it down to perfectly match the size.
Scissors if you're good, craft knives, guillotines, rotary trimmers and the like would probably be a touch better though. Just be careful that when cutting the foil strip (if there is one) you don't end up with pulls or rips.
Oh, and there's always a chance you'll be able to find reasonably accurate* dimensions for various banknotes on Wikipedia.
Have fun.
* Accuracy there will depend on the care taken by the respective article authors.