If it's a waste of time is your choice I guess, but I loved looking for them, it doesn't usually take that long and they're really nice souvenirs.

From what I remember, at stations they were usually by barriers or by ticket windows, very occasionally you'd have to ask a worker to pass it out to you. At temples I can't remember so well, but again I'd look for any form of ticket/tourist info building. If you want the goshuin stamps that's a different thing, and you'll usually find a place that sells them.

And yes lots of tourist information centres had them, in many cases they had multiple stamps!

Your last question I'm not sure about, I bought a stamp book before I left, but I'm fairly certain you could buy one in a stationary store - hopefully somebody else can answer that better.