Real black powder is classified by BATFE (the "feds") as an explosive. There are restrictions on production, transportation and storage of it. Most states have additional regulations on storage and transportation. The vast majority of us shooters are unaffected by these regulations as they involve bulk amounts - 50 lbs or more. The biggest impact on us is that insurance companies want more $$$ from a retailer if they keep it on the premises - that's the reason you can't find it in your local area.
Substitute black powders, such as Pyrodex, American Pioneer and 777, are chemically different and are not classified as explosives, thus they are more readily available.
Traveling interstate with real black powder is an issue if you are moving bulk amounts, depending on state laws. The definition of bulk amount varies from state to state. Your state transportation department should be able to advise you on what the rules are. Also, check the state laws on storage of explosives, usually specified in the construction codes, if you plan on storing more than about 5 pounds. Since you are not a commercial carrier (I assume) there should be no federal issue with crossing state lines.
Pyrodex RS will also work in your revolver, again with possible accuracy effects. Not optimum, but usable.