I bought mine from here many years ago when we had a weather gadgets shop, you can still get them though from other places:
http://www.strikealert.com/ProductInfo.htm
Might be worth a check on ebay and gadget sites from time to time to see if you can find a bargain, its a lot less reliable than expensive detectors like say a spectrum thunderbolt but it is also much much cheaper. Just keep it a couple of meters away from any electrical equipment to prevent false positives.