I've been in a number of comic shops (including several I'd consider very good), but I've never seen even one book of sample scripts.
Neither have I. (And I used to work in a comics shop.) The thing is, comic shop owners will only order something like that if they think there's a demand for it. There are
books on writing comics, but you're unlikely to find them in a comics shop.
Here are some suggestions:
The Best of Write Now!
Writers on Comics Scriptwriting
Panel One: Comic Book Scripts by Top Writers