I get mine from Classic Finishes in Norwich a specialist wood finishes supplier - There is likely to be a similar business in your area - Make sure you get 880 ammonia - Its the best in my experience for fuming.
This should be in Buying Advice.
You can get .880 NH3 from your local pharmacist. He'll ask you what you want it for, but provided you can convince him that you are legit and know how to store and handle it, he should be able to get it easily.
I got mine from Boots. Not sure how comparable it is strength wise but it has 'Ammonia Solution 9.5% w/w' on it. Just pick it up of the shelf or order online.
It worked well on the few occaisions that I've used it. I left the turnings in a covered container outside for about 6 hours (I think, didn't exactly time it).
I used ammonia froma local chemist after explaining it's use for a fumed oak coffer, it worked a treat , put dishes with ammonia in a tent made up of some left over floor membrane, made a bed of nails to hold pieces of the furniture up off floor, and also put in some test pieces in to test the colour, happy fuming