Obtaining KNO3

Potassium Nitrate is commonly called Nitrate of Potassa. It is sold as a fertilizer and may be available in your area as such. It contains, what is described as, quick acting nitrogen, along with K of course. The three main nutrients that plants need are Nitrogen (N), Phosphorous (P) and Potassium (K). The amount of each of theses elements are usually written on the bag in that order. The actual way the amount of the elements are numerically described can vary from country to country.
In the States a bag of fertilizer with 13.75 - 0 - 46 is a bag of KNO3 which is almost pure.
In other places this same bag will have 13.75 - 0 - 38.6 which is exactly the same stuff, ie. KNO3. The reason for the difference in the number is because in the former the amount of K that is in the fertilizer is described in terms of K2O percent, while in the latter the amount of K in the fertilizer is described directly in terms of percent using molecular weight figures. I have no idea why there are different systems in operation, perhaps it is more useful for the agricultural people to have it in thoses terms.
Sodium Nitrate may also be available and will have the number 16.4 - 0 - 0 on the bag or simply 16.4% N. The figure 16 may vary slightly (on low side) if the stuff is not pure.

Calcium Nitrate is sometimes available as a fertilizer and it is easy to make KNO3 from this if you have Potassium sulphate. If you mix a solution of the calcium nitrate and the potassium sulphate (in the proper proportions), calcium sulphate will immediately precipitate out as it is insoluble and you will be left with KNO3 in solution. You could also use KCl instead of K Sulphate, I will leave it to the reader to figure out the amounts/concentrations to use.

The following shows various ways to obtain Potassium Nitrate (KNO3) from the other, sometimes more available, chemicals.

KNO3 from Ammonium Nitrate and KCl
KNO3 from NaNO3 and other sources

Below is what you are liable to meet on a bag of KNO3

Info from Danish origin KNO3.
Bag has markings in 6 different languakes:
GB, F, I, D, NL and E. 
All markings indicate 13,5% Nitrogen and 46% watersoluble K2O,
exept GB, that lists also the K -content.

Heres the GB section word to word:
-------------------------------------
NITRATE OF POTASH

13-0-46
Low in chlorine
 
Total Nitrogen (N) 13,5%
of which Nitric Nitrogen (N) 13,5%
Potassium Oxide soluble 
in water (K2O)46,0% (38,2% K)

NET weight 25 KG
Marketed in the UK by:
Kemira Horticulture, Ince
Chester CH2 4LB
--------------------------------------

Product name in other languakes: 

(GB, NITRATE OF POTASH)
F,  NITRATE DE POTASSIUM
I,  NITRATO DI POTASSIO
D,  KALIUMNITRAT
NL, KALISALPETER
E,  NITRATO POTASICO

Also in bag generally:

POTASSIUM NITRATE (KNO3)
13-0-46
HORTICULTURAL GRADE
CRYSTALLIZED
-
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