Try the interweb if i hadgot mine formy fox in the tackleshop whitch will remain nameless it would of been 6 quid plus but off the net got three for the same money.
I use the same alarms my first batt cost me a tenna then i found them on a site for £1.20 eachso i know how u feel tank, anyway good luck dude just incase i will try and find the site i used for you mate.
Hi mate go to maplins its an electronical shop im sure you can find a store near you they sell those batteries for 99p each and there duracell thats where i get mine and because of the price its good to stock up on them,hope this helps you,tight lines.
When buying online, etc make sure you get the 1.5v LR1 Battery, as there is another battery the same size but 12v (used in some fox receivers) use one of these and you will blow the alarm and void the warranty.