Uberti makes an 1858 Carbine, Pietta might too. Needs a conversion cylinder to fire cartridges but typically fire .45LC. Some of their 1851's and 1860's can also be fitted with a shoulder stock to make what is essentially a short carbine.
the 1851's and 1860's can be converted to fire cartridges, but may be considered SBR's if converted and fitted with stocks. The 1858 should pose no legal problems being converted as long as the barrel is minimum length. Try the blackpowder forum if you're interested, brushhippie just posted a video with his stocked 1851 and the guys there would be more knowledgeable than I am.
I've seen 1851 carbines and one SAA carbine on gunbroker, but they aren't common or cheap and I don't remember if they were repros or original.