They are allomorphs but some people call them allophones (the past tense has them too: [t], [d], [Id]). Here's a good site. :-D
Learn English Pronunciation
=> Scroll down to where it says listen to the pronunciation. There are lists of nouns in singular and plural, pages of them. Click on the plural word to hear the allophones [s], [z], and [Iz].
All the best. :-D