I bought mine in Homebase in the section where they sell mops and cleaning products. White plastic bottle blue writing, labelled Household Ammonia. It was a couple of £'s.
From your bladder but ensure to boil it 1st , nah only joking. You could try a hardware store or pharmacy but as i understand you will be hard pushed to get hold of it in its PURE form because of the other uses it has and the way it has been used in the past for assault on others.
Jeyes Kleen Off Ammonia (This is what alot use in the UK)
This will do i would think as its just diluted ammonia (with water)