If you are looking for e-liquid suppliers, here are some reviews
However, if you are new to this all, read on...
E-liquid (e-juice) is a solution heated by the atomizer to produce vapor and thus simulate smoking. It is either contained in pre-filled cartomizers, or sold in bottles for refilling (10,15 or 30ml, but also available in larger quantities).
It's made up mostly of propylene glycol (PG) or vegetable glycerin (VG) base, or their combination, nicotine at desired level and
additional flavor. Sometimes other ingredients can be added (like distilled water to dilute VG).
Propylene Glycol (PG)
PG is a clear, viscous liquid
without any color and almost odorless, with a faintly sweet taste.
It is used in pharmaceutical and food products. Some people might be alergic to it (they are also alergic to other products that contain PG, like some toothpastes, soaps, lotions). Alergy symptoms are usually mild – problems with throat and sinuses, headaches and nausea.
your case, you should look for e-juice with vegetable glycerin base, and you
might be interested in e-liquid with organic flavors. (These folks are specialists here, and you should really try their e-juices. Virgin Vapor Review)
Vegetable Glycerin (VG)
VG is a colorless, nearly odorless, clear and viscous liquid with a faintly sweet taste. It provides more vapor but less throat hit than PG. Vegetable glycerin is extensively used in food industry, as a solvent, sweetener, preservative, thickening agent, filler in commercially prepared low-fat foods etc. It is also used in medical, cosmetic and personal care products.
Since VG is rather thick, experts say that 80-20 PG-VG ejuice is good both for you and your atomizer (cartomizer). However, distilled water can be mixed in to dilute VG properly. If we stay with PG-VG, I've seen them sold in 80-20, 70-30 or 50-50 ratios. Some flavors are better suited for PG, some other for VG.
Usual variations of nicotine levels are 0-6-12-18- 24mg/ml. Somewhere you will find it sold in cartomizers at 4-8-12-15-20mg. And you will also find bottled e-liquid sold with higher nicotine levels. Although such high levels are not recommended for direct use and are normally sold for the purpose of mixing (dilution), some heavy smokers use them. The highest I have heard someone use is 36mg.
This is the right place for a bit of advice - if you are, say, a pack a day smoker of light cigarettes and are told to take 12mg nicotine level cartomizer or e-juice, you might want to try 18mg too. It somehow seems that everyone is obsessing about reducing nicotine levels, but I don't think you should worry about that right away.
Most important thing for you is to make that switch from tobacco to electronic cigarettes. That's why you need to find an e-liquid with the right tobacco kick in the throat. You can worry about nicotine level later. Anyway, according to this study, much less nicotine is absorbed from electronic than from tobacco cigarettes.
Of course, what I say about nicotine level preferences can't apply to everyone equally. You might find your 12mg are exactly what you need. Or you might feel better with 18mg nicotine during the day and 12 mg in the evening.
As for as eliquid (e juice) and all those who prefer pen-style e-cigarettes, your daily consumption of it also largerly depends on what kind of vaping equipment you've got.
Standard pen-styles with standard battery voltage and atomizer resistance are an economic solution. FYI, as an ex smoker of pack-a-day light cigarettes, my daily consumption of e-liquid is about 1.5ml of 18mg or about 2.0-2.5ml of 12mg strong e-liquid.
Food grade flavors are used for ejuice - most common is tobacco flavor. It comes in variations that imitate the existing famous brands of cigs and tobacco (Marlboro, Virginia, Camel etc). If desired, any other or any additional flavor may be added - vanilla, menthol and some fruity flavors are most common.
There are many other and there seems to be a new one coming every day. I like UK Smoker's Halo liquid (see below), Italian Flavourart (PG-VG mix, fruity flavor, nice smell and aroma). I used to vape Chinese Dekang in many variations (PG, PG-VG fifty-fifty, American mix, Chocolate PG, Vanilla VG, you name it. Some people complain about it, but I have never had any issues).