In the UK glycerine is available in pharmacists for ~$2 per 200ml. https://www.boots.com/en/Value-Health-Glycerin-B-P-200ml_852...
The devices themselves can be bought from overseas - AliExpress has many variants, some for <$10.
At that stage, the 'safety of e-cigarettes' is equivalent to the safety of inhaling atomized glycerine. Stage smoke.
Flavourings - you can look for things which are used in aromatherapy (e.g. menthol crystals), but you'll probably want to study whether any chemical changes can occur under heating.
Pre-mixed nicotine is a difficult one. I don't use it so I'm not sure if you can acquire it from trustworthy sources.
I would not want to fall into the habit of using e-cigarettes produced by tobacco companies such as the 'vype' disposable (British American Tobacco). They have a history of producing methods to increase the addictive potential of cigarettes, ramping up nicotine levels, to cheating on tests, using ammonia to produce 'freebase' nicotine, etcetera. I wouldn't be surprised if the same bag of tricks comes out in the e-cigarette market.