Umm you mean the black jack?? That's not like a club at all moron.
-I'm from Jersey- I'm an beep

The round ones are blackjacks, the flat ones are "slappers"
here is a good link, with pictures and descriptions of each kind.
http://www.donrearic.com/sap.html
Due to state law, they are calling them "leather impact weapons", "sap", and "leather billy" instead of "blackjack" and "slapper"
A fine selection of "slappers" can be found at "Boston Leather"the last "reputable" company making them since "Binachi"
http://www.bostonleather.com/UMSS.cfm?page=page-index.cfm&template_code=Search.Do&category=24
Blackjacks can be found here:
http://www.selfdefenseproducts.com/police/impact.htm
OR
Save some money, and get yourself some paracord, a lead ball sinker, and make yourself a "slungshot". All you need to do is learn to tie a monkey's fist knot.
Slungshotthis will cleave bone:
http://i10.tinypic.com/2r7oaz6.jpg
Tying the "Monkey's Fist Knot:
http://www.igkt.net/beginners/monkeys-fist.html

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And just how did the guy searching Nick towards the end - when they get to the test site for the final confrontation with the general- miss the thing in his breast pocket? Pretty shaky..

When I was working as a cop we used to call that type a "Slapjack". They work wonders.A "Sap" is what William Bendix used on the Jap soldier in "Guadalcanal Diary".

Saps are carried by Animal Control Officers in many places, including New Jersey. They're powerful weapons flat piece of lead coated with rubber. If you slap one into your palm, even playfully, it hurts and numbs your hand for a few minutes. It's hard to imagine why Nolte has to whap the FBI guys several times to quiet them down.

i've heard it referred to as a "cosh"

Nolte's weapon is in fact a sap. Most likely a Bucheimer "Texan" model as it is identical to the one I own.
Don's site will tell you all you want to know. However "blackjacks" are not outlawed nation wide. They may be restricted or banned in a paticular place but no federal law exsist that makes them illegal. The North Carolina Highway Patrol issued then up until the early 90's IIRC.
They are awesome weapons. I carry a sap or blackjack everyday on duty.

I lost my shirt playing flapjack in Vegas. (And now I'm really
cold
)
Here's an article about saps and blackjacks that I found online a few weeks ago - more than you'll ever need to know
http://www.donrearic.com/sap.html

I have such a weapon. It was my father's. He was a constable in a small township on the edge of two large cities in the late fifties. After his death in 2001 my mother gave me his sap and his Colt .45 1911 A1. My brother got dad's badge though. I would love to have received it too. I do have his card authorizing him to carry a concealed weapon.

Nolte's weapon is a particularly large sap, which is flat, leather-covered, and filled with lead or lead shot. A blackjack is also typically leather-covered lead, but is cylindrical and therefore delivers a blow to a smaller area, making it deadlier. For a quick, close look at a blackjack, see the scene in The Untouchables in which Kevin Costner and Sean Connery meet Andy Garcia for the first time, at the police academy. To "see what he's made of," Connery goads Garcia with an ethnic slur, in response to which Garcia draws his revolver; Connery in turn pulls out a blackjacka symbolic response of course, since a blackjack is no defense against a revolver.
Saps are still carried, in accord with regulations, by detectives here in NYC, and elsewhere I imagine, whereas blackjacks were outlawed, even for cops, long ago (which doesn't mean, of course, that some cops don't carry them). For more on saps and blackjacks, try this: http://www.donrearic.com/sap.html
As to where/how to obtain these weapons, is there anything you can't get on the Internet?
And a flapjack is something to eat.

I also thought they were called flapjacks like the OP but I've ALSO heard them be referred to as SLAPjacks!! And with these other chaps insisting they're called BLACKJACKS now I don't know who to believe or what the hell the weapon is called! Haha.

You're rude for making such a comment to "flapjack". He was not far off. Obviously, you didn't know that the flat ones were sometimes called flat saps.
A blackjack is completely different than what Nolte used. A blackjack has a round end and is very lethal. The flat sap is flat, with lead stitched into the leather - with a handle.

I see them at gun shows and weapons stores now and then too
Illigitimi non Carborundum

Ya really big dummy, videoguy-5; I don't think there are any educational standards in the world that would list the knowledge of blackjacks as a requirement for being considered intelligent. Maybe wherever you come from, I don't know. But you must be smart (?) because you have "read" early cop movies, and you can use two different words for the same meaning (grip, hold). Move to the front of the class, Einstein.

No cop I know calls it a flapjack. The only people I've heard call it a flapjack are people who are confused.
To be correct, a blackjack has a spring handle with a weight on the end, then all wrapped in leather. It is round.
A sap (sometimes called a beavertail) is flat, made of leather, with lead shot sewn into it.

Blackjack, I win!!

Mmmmmmmmmmm flapjacks.

Actually, within the above reference, flapjack and blackjack are interchangeable.

Ya big dummy. . . it's a "blackjack". Didn't you ever read cheap or old crime novels or even early cop movies. It was usually a small lead filled ( or BB filled) sack or "sap" covered in leather with a handle to grip or hold.
Yes, sad to say, they are still legally available mail order or speciality stores.