It allegedly comes from a public toilet on the South African side of the border at Beitbridge.
Be a BYO paper toilet 
I guess Zim dollars aren't worth much for people to actually use it as toilet paper. 
They changed their currency 100 billion dollar note replaced by the new $10 note WOW...
So using old million notes would be cheaper than real toilet paper!!
http://christopherreichert.blogspot.com/2008/08/would-you-like-to-see-50000000000.html
Then they gave up and abandoned the currency completely. By the time the ink dried the paper was worth more than the banknote. When they gave up the exchange rate was around 500 trillion Zim dollars to the US dollar.