My dictionary says AmE is risors and BrE is ri'zo:s
I have heard an American, I don't know where he is from but sounds from Texas, saying it with z sound.
Do Americans pronounce it with a z?
Is every /s/ become /z/ when it's between two vowels?
'S' is often pronounced z between vowels but there are exceptions when it can be pronounced either 's' or 'z' or when it is pronounced 's'. Most Americans pronounce the first 's' in riser/ risers as 'z'.