Where to get Alarm battery backup ?

got a 480xv viper alarm, car off the road for now, went to get into it, battery was flat, no probs, it has been 2 mths, but the alarm is also dead as well, wouldnt unarm etc, so undid with the key, alarm never went off as the car battery is flat flat flat.

i presume once i charge the battery the alarm would go off as it would re-instate its armed state, but thats no good when the battery is flat for it being broken into etc.

can see alot of the viper bits in the states goinf cheap,cant see in the uk much,if there is bet its topp £££

anyone any pointers ?

i got similiar prob but not with viper alarm, my bat when dead when car was off the road but even now the battery if fully running again me alarm doesnt sound or beep when i lock/unlock the car.