The maple syrup actually is the "SUBSTITUTE" as the "original" Master Cleanse was made with sugar cane juice but because it is not readily available in the states Stanley then used Maple syrup. In many Latin American and Caribiean countries sugar cane juice is quite common. I can only speak for the Cuban name for it which is "guarapo" and it's commonly found in South Florida's Cuban communities.
I found it in the states in an international farmers market with a large Indian population but it was too far to drive and too expensive. Sugar cane juice does not keep even refrigerated for more than one day at best so you have to have it fresh made daily. The one thing I will say is that I like the taste muuuuuch better than the maple syrup and specially using limes instead of lemons. To bad that it was so expensive and sucha far drive for me.
Check around maybe you find it's also common in Chile. BTW if you read the section in the book for diabetics you'll find that Molasses is an acceptable substitute and it has beaucoup minerals and lots of iron. The spanish name I know for Molasses is "Melao de Caña".