I have just been reading posts about these.I used to take those sheets and get them wet and no good etc etc. Then i got them laminated pocket size,thought i was onto a good thing.
Then i discovered the one you wear around your neck.Fantastic.It is the size of a credit card or small calculator. You set the days rate and then you just press the amount of what you are buying and it will tell you how much the item is. You can use anywhere in the world....just put the countries rate in and only change it when the rate changes.
Slide the bottom open and it also becomes a calculator.
It is from Korjo and sold in bag shops, travel shops, and i have even seen one on ebay.
They are around $20.