I don't know if anyone will send you the source, but I can give you some advice.
It is best to buy SN in chemical stores that sell many products (not only SN), or possibly in meat stores (SN is mainly used for curing meat).
Search SN mainly in Europe and Asia.
Don't necessarily use only English to search for SN.
You can also use slightly less popular languages.
I think you can easily find the source. Just spend some time on it.
However, if you live in the USA or Canada, for example, it may be a little more difficult.