I THINK the part you seek is called the Bolt Spring. gunpartscorp.com has an exploded diagram of the Uberti Navy, which shows the spring, but they do not list one. Neither does Dixie Gun Works. Brownells lists the bolt, but not the spring.
So you have a couple of options- first, email all 3 above, and ASK if they have a spring. Sometimes they do not list ALL the parts they have (small quantities) And consider that a Colt spring will possibly be a fit. Second, find a gunsmith. Making small springs is a common task for a true smith. Third, get some spring stock, file, and do some reading on tempering springs.