christ, your lucky you aint lost a fert yet, 393 is the number for your collar batteries, a duracell 9volt will do for the locator, I tend to buy batteries in bulk from evil bay, so they work out to around 20p each, then use new batteries in the collar on every outing, considering the value of a collar is around £50, its a small price to pay (40p) each time
just saw you'vre got the mk3m, so 394 as stated by others