"Slide holster" as in belt slide style? As in a holster that has slots roughly at the left and right edges for the belt to pass through, as opposed to a single loop centered on the holster body or loops at roughly the left and right edges?
Given the fact that the Nagant is not a highly popular model there are few holster manufacturers offering a variety of models. There are some that make replicas of the original military issue holster, but those that I know of for the Nagant are not belt slide style.
Check the makers of generic holsters - there are many leather hosters as well as nylon/cordura that a Nagant will fit into with a reasonable fit. Most likely you will have to choose a more common handgun that is similar in size to the Nagant and use that as your search criteria.
I support your decision to carry the Nagant. Do not let folks try to tell you the .44 Russian is an underpowered round - unless they are willing to stand in front of it and let you shoot them to prove their point. Do not let folks tell you that the 20#+ trigger is too much - instead remind them that when you can handle the Nagant's trigger you will be able to handle any of the lighter triggers without a sweat. Check out this video on the Nagant: http://www.youtube.com/watch?v=kIePW1wYZds