Brou - indeed, the FN 57 pistol is now available as a non-LEO/Mil variant, but why bother as you said.
Regarding the "cool" factor of the P-90 ... to me, ergonomics of the reload are backward, and without the suppressor, this rifle is too easy to "muzzle brush" and get ugly. Opening a door with left while trying not to cross over yourself is something that will require practice. Also, drop a half-full (or was that half-empty) mag on the deck, and well, might as well have thrown it in the trash.
If you want performance of that type round, go with the aforementioned 5.45x39 (Alexander Arms' Genghis upper perhaps) or else talk some gun smith into making you an upper in 5.7 Johnson Spitfire and use the Oly Arms 30 Carbine magazine conversion.
Or talk a smith into building you an AR in 7.62x25 (just finished #3) and load with the .223 Timbs. That is the factory name of the 7.62x25 with the sabot. Don't get me started on the renaming bit, to me it should be called the 7.26x25 SLAP, but that is just my opinion [;)]
Back to the original topic: Stewart Galleries is one of the two places that sells the sabots. They will provide load data for several 30 caliber rounds to use with these little gems. Remington now sells them factory load again, called the Accelerator, they USED to offer them in 30-30, 308 and 30-06, now IIRC only in 308.
Stuporman - shooting the sabots through the can could lead to some bad voodoo, if the petals open inside the can ... which I suspect they will. The term baffle strike come to mind ... ouch.