This used to be purchasable in gardening centres. It might still be so, but I wouldn't be at all surprised if it wasn't, because of stiffer regulations. My old school friend used to buy it from such places when he was making his illicit fireworks. The other materials he bought from specialist chemicals suppliers such as Aldrich but, again, such suppliers don't sell to the public anymore. And if you are thinking of buying in order to make your own fireworks, shame on you.
Where are you Phillip Harris and Griffin & George. I used to buy this Potassium Nitrate and other chemicals in my teenage years from these companies by mail order with no questions asked.
Without a bona fide purchasing order and the correct credentials Sigma Aldrich, Fisher Scientific and the others will send you packing as shammydodger implied.
Potassium Nitrate is a extremely dangerous oxidant and should not to be messed about with. Put the idea out of your mind.
You also need to be aware, that in the current climate of anti-terrorism laws, if you were caught trying to buy this stuff or being in possession of it, you could end up being interviewed by the police and locked up. It wouldn't be necessary to charge you for any crime.
Ah, what memories this brings back of my teen-age years when I could buy potassium nitrate at - would you believe?- a chemist! Yes, chemists actually sold chemicals in those days. I could give you a list of other chemicals I used to buy to make more sophisticated fireworks than were possible with mere gunpowder, but the anti-terrorist brigade would probably arrest me.
I often used to buy potassium nitrate and other chemicals at my local chemist. I once wanted to make some Fehling's B solution and had to place a "special order" for sodium potassium tartrate with him!
Other stuff I used to get at my local ironmonger's including "Muriatic Acid" (HCl) and "Blue Vitriol" (Cupric Sulphate). One of the most versatile chemicals he sold was tins of "Waterglass" (Sodium Silicate). The ironmonger didn't have a clue about the chemical names of these substances.