Cabela's has Ten-X .32-40's, at the already mentioned $42 for a box of 20; Buffalo Arms lists .32-40's at $35 for 20.I worried any ammo I locate will be too hot for my old gun, but I would like to see how it shoots before investing in reloading dies for it. I'd prefer not to shoot BP if I can avoid it, as this thing is a real PITA to take apart and reassemble.
Smokeless loads for the .32-40 were available as early as 1895, and Winchester loaded a High-Velocity version for the Model 1894 from 1905 until 1941. Manufacturers stopped chambering rifles for the .32-40 some time in the 1930's or 40's, except for a limited run by Winchester in the early 80's. Any commercial ammunition will be perfectly safe in your rifle, provided yours is safe to fire in the first place. Reloading will, ultimately, be the less expensive route, depending of course on how much shooting you plan to do.