willie, I don't have a real source for Tonkin bamboo, but plain old moso planks from franksupply.com have never let me down. They have a lot of different stuff, and the staff is pretty knowledgeable if you need to phone them.
I think trade names get in the way with bamboo, like they do with tropical woods.
From what I understand, tonkin cane is a very small diameter, tough, straight-sided, small node bamboo (or relative).
Tonkin bamboo (capital "T") is a strong bamboo that comes from the Gulf of Tonkin area. Not sure the actual species, but my memory tells me it is most similar to Taiwan Bamboo. Both are larger diameter and much thicker-walled than Moso. Taiwan bamboo is used whole for highrise scaffolds in Taiwan.
What Frank Suppluy calls "Calcutta" bamboo is also called "tropical bamboo" by other dealers and has almost no hollow center. Another specie called "tre-gai" is a lot the same, close in size, more hollow, but still a really thick wall.
I bought what Three Rivers Archery called a Tonkin Bamboo backing for my first few backings, but when I bought from franksupply.com, it looked, felt, and worked exactly the same. I wouldn't be afraid to use any decent diameter bamboo (Tonkin, Taiwan, Black, Tortoise shell, or Moso) for a back, but would maybe look for thicker-walled specie for a belly.