Is there any site which provides clear and understandable information on what banknotes are currently in circulation and which ones have been discontinued? I can't seem to find anything.
That would be interesting. I have this list containg current currency - http://www.worldatlas.com/aatlas/infopage/currency.htm Let's see what other members can provide. I will keep searching.
The MRI Bankers Guide is a printed work that is used by banks for current banknotes. There is no online version. It is expensive but you can sometimes find recent (year or two old) on amazon or Barnes and Noble or other used book sites. The MRI website does have limited information about new notes.
I agree with Lettow, In the UK I obtain used ones (free) from my local Post Office which acts as a bureau de change.