You used to be able to get it at hardware stores and old fashioned chemists - try a local independant hardware store - I believe that - in common with chlorates - the stuff the sell to the public contains explosive inhibitors so would require refining if you are thinking of making gunpowder!
you can get it in gardening stores: potassium nitrate. If you want it pure for food purposes you need to talk to a food industry supplier,. Try Fisons. Or for chemical purposes either Aldrich or Lancaster Synthesis would supply it.Both have websites.