I'm assuming you mean 50mg nicotine and not 50% nicotine. 50% nicotine would be 500mg and would qualify as a hazardous material requiring special handling procedures - definitely not for the casual mixer.
We're not usually looking for the cheapest possible nicotine regardless of quality and flavour. Cheap nicotine will likely impart a strong 'peppery' taste to your mixes, particularly bad if you are mixing for delicate flavours. If you are mixing at 12mg, 1 liter of 48mg nicotine will make 4 liters of e-juice - so you are going to be living with that taste for a long time.
I've personally used nic base from Hydavape in Arnprior with good results (you can order online & they ship anywhere in canada). 1 liter of 48mg is $75 canadian. 1 liter of 100mg is $125 canadian. I transferred all of mine to 100ml blue glass bottles that are stored in the freezer until needed.
Do NOT keep it in a cupboard - it will oxidize and basically taste like crap.