No need to go farther than wall mart, canning section.
I've made eliquid since July 2012, for my family, friends and a few loyal locals.
The concentrates used to flavor eliquid can be bought online at many places (The Flavor Apprentice, Nicotine River, RTS Vapes, Wizard Labs, Capella, LorAnn's, Flavor West, Bull City, Liquid Barn and many others). These are used in foods, eliquids and I am positive they will work best in a casing. They are basically very strongly flavored pg (propylene glycol).......U will likely need to dilute them with distilled water, possibly as much as 50/50......insert them in a misting bottle and there u have your casing. 100's of flavors are available.........I'd avoid buying menthol crystals if you are a menthol smoker like myself, instead buy LIQUID menthol suspended in pg.
I also have liquid 100 mg nicotine and will likely add about 9 mg nicotine to my casing to enhance throat hit and nicotine levels, as I bought all Virginia Flue leaves ( 4 types) and no burley or throat hit types.