Blown main fuse

I have a 2014 kia optima ex I installed after market
subwoofer with amplifier and interior ledglow
colored lights I took it to kia and i told them that
the steering wheel would go in and out and the
warning light would go on and off they said that
they could not do any more testing until I got a new
battery and the only thing that showed up was that
The battery wasn't getting a charge so I change
batterys and it can back but by the time it came
back the warranty was up now the main fuse that's
connected to the battery is blown the steering is
out and the check engine light is on and now the
battery light comes on and my neighbor who is a
mechanic diagnose the problem with his scanner
and it only shows that the battery is not getting a
charge also but the guy at Kia said I had to replace
the full harness of the car which would cost over
$400 also if I can just replace the main fuse to the
battery positive terminal where can I buy that
online or in stores