Where to buy a new battery - Advice please

[Mud]

I have searched on the forum for a thread with little avail, if this topic have been covered please just refer me there.

So the bike have been standing for a while due to the addition to the family and being a new year and new opportunities, the bike's battery is kapoet.
Where are the most affordable batteries to be purchased? Is it just me or have the batteries become really expensive of late?

Any help will be greatly appreciated.
(It is for a Triumph 1200)

Thanks

 

[Skeltonsc]

 

[Mud]

 

[Tr0jan]