I have never heard of terrorists using Potassium nitrate. It is ok to buy it over the internet.
Usual response to this question (you are the latest of many many many people to ask this) to buy from Garden Direct. Searching, either this forum or google will come up with their website. Who knows, perhaps they are within driving distance and you can buy from the store.
Don't pay more than a few pounds per kilo, you are being ripped off in an epic way if you do.
Have you read this entire thread?
It is a decent task but you will learn a lot if you do. You will learn things you would never have thought of asking.
Good luck with the special branch anti terror cops, they are known to be watching this forum.
Kinda joking, they might be, but they are kind to us. They are our friends.
The monkey leaped off it's sunny perch and flew off into the night sky.