Bactinettes (pro. Back-ti-nets), they're kept in the fridge and the only live bacteria thats likely to work. Does depend on good storage by the lfs though.
Ammonia Alerts are available in most lfs but only a few stock prime or amquel (only one in hampshire/west sussex that I know of) Easily available mail order though and probably cheaper. Prime is expensive as a bottle of dechlorinator until you realise it also deals with ammonia, nitrite and nitrate toxicity and is so concentrated that 5ml treats 200L of water.