Well, someone on the BVMS (British Victorian Military Society) message board posted this which they got from Ten-X Ammunition -
We were able to make up some 577-450 Martini-Henry blanks by forming 24ga CBC brass shells. The price is $4.49 each and they are packaged 20 rounds per box. There are a limited number of those blanks left.
However, that is a very
steep price for something you can make yourself, without any need for loading dies! (Of course, for a blank, all you need is some black powder, primers and suitable wadding which will not form a dangerous "projectile" - wadded up toilet paper or kleenex works fine. A simple rod and pin de-priming tool will easily punch out the spent primer, and a new primer can be seated into the primer pocket by simply placing it (cup up) on a good firm surface (an anvil is ideal), setting the empty cartridge case over it and tapping it down with a dowel or the like. Pour in your measured charge of black powder and tamp in the wadding.
Formed brass (from 24 gauge CBC brass shotgun shells, just like Ten-X used for their blanks) is available from this chap for only US$41.95 for a box of 25, postage included within the US - http://britishmilitariaforums.yuku.com/topic/15223/577450-MH-and-577-Snider-Unprimed-Cases
(I know Martyn personally, and have shot with him - he is a gentleman! The "testimonials" posted in this link speak for themselves.)
Believe it or not, it is even possible to produce a safe blank which actually looks like a complete cartridge, complete with bullet seated:
(Not that this "look" - and the trouble to achieve it - is at all necessary in a single-shot rifle ..... but these are blanks I loaded for use in .45-75 (top) and .45 Colt (bottom) lever-action repeating rifles .... for which blanks for multiple shots must feed through the action, and thus must effectively duplicate the shape and dimensions of a loaded cartridge.) The method for making such blanks is detailed in this forum thread (and at the link mentioned within it) - http://www.cascity.com/forumhall/index.php/topic,43872.0.html
As you will read if you follow the link, this type of blank has been approved for blank firing "re-enactment" purposes by the Parks Canada "firearms safety police" - and so ought to pass muster anywhere
! The "bullet" is formed of a tapered strip of rolled soda-pop carton cardboard, dipped in wax. When the blank is fired this "projectile" provides resistance (necessary so your shot goes "bang" rather than "woof") but then either blows apart completely within a short distance of the muzzle, or simply uncoils and drops to the ground within 10 or 15 feet.