Check rural feed stores. KMnO4 is sometimes sold in them to treat water wells.
Note: you'll probably need to buy a five pound bag.
In the UK pharmacists or chemists sell it in bottles of 25g, if you ask (it's not usually on the shelves). 25g is plenty. A little goes a long way. 5 pounds would be over 2000g, a slightly scary amount to have.
As a fire-starter I've found it to be a bit temperamental. Also it's not particularly convenient to carry glycerine because it needs a small, water-tight container (you really don't want it to leak) which takes up a lot of room for what it is.