Everyone from Nerf to Sideshow Collectibles will happily sell you a replica of Thor's mighty Mjölnir Hammer. But if you want it to work just like the one in the comic books and movies do, you'll need to do what YouTuber Sufficiently Advanced did and build one yourself.
Not only does his creation look spot-on, it also features a built in battery-powered electromagnet (borrowed from an old microwave) that only turns off when it recognises his fingerprint via a thumbprint reader on the handle. When placed on a metal surface like a manhole cover, nothing short of a crane can lift it.
Given all the electronics secretly housed inside this functional prop it probably wasn't a cheap build, but it probably wouldn't take long to recoup the cost if you could bet enough people they weren't strong enough to lift it. [YouTube via Taxi]