Are you using the plug-in? As one plug-in in the room most used by the cats should cover the whole of a three bedroomed house, for four weeks. Also look at www.fabcats.org for great other tips on how to reduce their stress. All the info on their has been produced by a panel of veterinary feline behaviourists.
The answer is the on-line pharmacies, but you need to check round as they alter their deals all the time.