Have you tried GunBroker? I have found a fair amount of ephemera here. Worth a search or two. Someone with Uberti manuals might also have something unlisted, so worth asking.
You might also look for Uberti 1849s for sale on the off chance someone is offering one in box and might have the manual and be willing to make you a copy. Not original, but the information would be there.
~Secret Select Society of Suave Stylish Smoking Jackets Founding Member ~