I can't find the rule regarding how this applies to nicotine from other sources?
You won't, because as I explained, things are regulated based on intended use.
If a product - ANY product, including one with nicotine as the active ingredient - has an intended use (ie. marketed or commonly used as) a food, drug (medical) or cosmetic then it automatically falls under FD&C Act of 1938.
If it is intended to be used as a diet supplement, it falls under the FD&C Act amendment passed in 1994.
If it is a tobacco, tobacco derivative or intended to be used as a modified risk tobacco product, it falls under the FSPTCA of 2009, which amends the FD&C Act.
So, it is the intended use of the nicotine-containing product that determines under which regulations it falls, not the source. There is no regulation that currently covers recreational use of nicotine extracted from plants other than tobacco, because it is neither a food, medical drug, cosmetic, dietary supplement, tobacco product or MRTP. But you can be pretty sure that something would be passed pretty quickly giving the FDA regulatory power over it should it start to be widely distributed to the public. Probably the same regulations that will eventually cover another popular drug recently legalized in some States. Or they'll just keep it under tobacco product because most nicotine comes from tobacco and the intended use is identical to tobacco.
But it will still be the marketed, intended use of that non-tobacco nicotine product that determines which regulatory act it falls under.
As an example, cranberry products and cranberry derivatives are not regulated based on the source being cranberries. They are regulated based on the intended use as marketed. If it is being marketed as a fruit or fruit juice beverage, it falls under FD&C Act as a food. If the manufacturer starts advertising the juice as a remedy for urinary tract infections, that product (not ALL cranberry products, just the one making the claim) will put itself under FD&C as a drug treatment. If a company extracts the abundant vitamin C from the cranberries and sells it, it becomes a dietary supplement and is regulated differently than as a food or drug. Just because products share the same simple cranberry source doesn't dictate how it is regulated. Vitamin C also comes from other fruits and vegetables, but each vitamin C product is not regulated based on the source fruit or vegetable, but on the intended use.
What dictates the regulation is the marketed intended use, but there is no "written rule" for that. It's just what happens under all of these Acts.