Sedum is a genus of plants with many species. Quite a few of them are succulents with leaves very different from what you see on the 'Autumn Joy' cultivar. As lip1976 mentioned, the species name is spectabile. 'Autumn Joy' is one cultivar. As it so happens, it is a very popular and successful plant, often appearing on top ten perennial plant lists when you see articles in magazines. It dies to the ground in colder climates, but comes back in the spring, so it grows in a wide range of climate zones.
Most nurseries should carry it, but keep in mind that 'Autumn Joy' is a fall bloomer, and flowers sell, so nurseries tend to stock what is in bloom. You might not see it in a nursery till summer/fall when people are going to be attracted to the flowers.
Many cultivars have been developed from Sedum spectabile. 'Brilliant' is another popular cultivar. A similar species, sedum telephium, also has many cultivars. If your nursery doesn't carry Sedum spectabile 'Autumn Joy', it may carry other cultivars of spectabile or telephium. As far as I know, they should produce similar flowers to 'Autumn Joy'. The colors while in bloom are probably different, but that won't matter since you are using the dried flowers. They should have a similar look for making trees for train layouts.
More than some of you may want to know, but it may help.