Where can I get a replacement battery

My phone has a problem after an encounter with water. it wasn't wet but then the phone went off and only comes on when plugged in. I researched and found out that its likely to be a battery problem but I cant get any battery for my phone online and I don't know why. I have tried Amazon but it just shows me the phone as a whole does this mean that I can get a replacement battery? or is this really a battery problem