If you are referring to people who sell newspapers and magazines*, then "newsagents" is the normal plural form of "newsagent": In my neighborhood there are two newsagents.
If you are referring to the newspaper shop that is owned by somebody who sells newspapers, then the possessive form "news agent's" is possible: I'll meet you at the newsagent's at four this afternoon. = I'll meet you at the shop the newsagent owns at four this afternoon. You should realize that "newsagent's" means "the place that belongs to the newsagent." It doesn't express any kind of plural.
* I never hear people use "newsagent" in my part of the world. I'd expect to hear something like "news vendor".
Thanks for the confirmation, Mr E. Once I found the compound "newsagent" in our dictionary, I figured it was commonly used in BE. I'm sure I've seen the word in print over here, but I don't think I've ever heard "newsagent" in a conversation with anybody.