APC batteries look easy to replace, but I couldn't comment on their availability.
I was fortunate to aquire a number of APC 500VA units a while back. A couple had dodgy batteries, but they are a "standard" sealed lead acid battery. Norwich is fortunate to have a specialist battery supplier, and the required units were in stock for some £20 each.
If your Belkin UPS is still serviceable, I would certainly recommend seeking out a replacent battery - It shouldn't be too expensive, and would be cheaper than purchasing a replacement UPS.
Regards, Paul.
Most the batteries require regular re-conditioning which usually involves a light or a fan, a trickle charger and a lot of spare time.
JT