Most liquid nic comes from China, some comes from India.
The source is likely to be chemical and pharmaceutical suppliers who specialise in bulk vegetable extraction and processing operations, not cigarette companies. Some might be owned by cigarette companies. This is a different business from tobacco growing for cigarette production.
The question of purity / quality is addressed by the buyer opting for the most expensive product they can afford (or the cheapest, depending on their aims). Just as with any other product, there are offerings at all price points. If you buy cheap e-liquid then only cheap materials can be used - it's as simple as that.
The purity is quoted as 99.4% through to 99.8% pure. There is more risk from the extraction solvents remaining than any other contaminant. The common solvents are kerosene and hexane, and it is likely that most extraction factories will use kerosene as as it's cheaper, easier to handle, and re-usable. Clearly you don't want any solvent to remain.
Pharmaceutical manufacturers in the West will use the best purity they can get, and it isn't cheap. We also know of a large-scale trade e-liquid manufacturer on the US East Coast who uses this grade, and it is so expensive that one employee even thought it was synthetic before it was pointed out to them that they could multiply that high cost by several times before it approximated the cost of synthetic nicotine.
The only known way to buy a high-quality e-liquid at this time is to buy an expensive product from a large-scale vendor who has a massive amount to lose if they are discovered to be selling poor-quality/contaminated products. To supply a good product costs money for high-quality ingredients and for testing of the finished retail product, therefore it is impossible to sell a high-quality e-liquid cheaply. What an individual vendor says about their quality is irrelevant, most of them don't have a clue what is in their products (Dr Farsalinos just proved that 7 out of 10 vendors don't know what is in their products).
Of course, people can spend what they like, it's their choice, and people have different priorities. You can buy a cheap or an expensive refill, as you wish. Please don't expect cheap offerings to be of high quality as that is simply impossible. Of course, there is a separate question whether all expensive products are high-quality - but in this case you should expect to see lab test results as that's what you're paying for.
