E'en tho they must be made somewhere for the shooters' models, I do not know where to get the "quality" 1858 guts.
I might conjecture that a fellow could fit and polish existing ones, then case-harden them.
I used to know a fellow who used factory guts as templates and made his own internals out of tool steel!
With todays supposedly affordable benchtop CNC mills that might be an interesting concept....
Now oddly, along the same line of thought , I discovered one can take "standard size" "copy of colt Model 1873 Single action"
hammer, trigger, bolt, and hand and they drop RIGHT INTO an 1851 or 1860 and make it soooooo smoooooooth ....
of course the hand must be modified, and the hand & bolt fitted, and they cost a lot more than soft italian C&B parts but...
yhs
prof marvel