So far I have found the least expensive place to buy a Towa is if another member is selling theirs, rarely there is a good deal on eBay, but sometimes you get lucky. New I have found Superior and Amazon are pretty close in price last time I looked.
Yes Superior has a tension chart, but they are for DSM as it is focusing on top tension. Superiors does have a thread chart that does give needle suggestion for each thread that is useful.
I agree with Cagey, there are several posts about the Towa and setting your bottom tension, and Linda is a huge wealth of knowledge that shares so freely. Li