There are several ways to approach this question. The obvious is if the rifle says black powder only then that's all you should use in it. Anyone who would say otherwise violates the manufacturers warning. Our suit happy society and liability lawyers have caused this.......and, the majority of folks don't know spit about BP cartridges, (or that they even existed), the firearms that used it or anything about internal ballistics.
I own several rifles manufactured before smokeless powder was around and I shoot all of them with a smokeless load sometimes. Not often, but sometimes. Trick is to know what you're doin'. I would suggest two books to you. The first is Frank Barnes "Cartridges of the World" and the second is "Big Bore Rifles and Cartridges" published by Wolfe Publishing of Prescott, Arizona. Both books give loads and loading methods that are safe to shoot in many of the old and old style firearms, providing the firearms are safe to shoot. Another good book would be the SPG Black Powder Cartridge Loading Primer. It will certainly get you headed in the right direction.
I have been shooting a Shiloh Sharps, Montana Roughrider in 45-90 for 12-13 years and have never fired a cartridge loaded with smokeless thru it. All the individual componets are available for reloading and you won't beat black powder ballistics by much unless you severely overstress the action, which is undesireable in the first place. My first and best suggestion would to acquire the necessary componets and "roll yer own".
I can't answer your question about the ammo Cabela's sells for the 45-90 but one would think that it is loaded to pressures suitable for old rifles. Again, given our suit happy society, I cannot imagine a firm such as Cabela's would open themselves up to an obvious legal problem by selling ammo that is likely to blow up an antique firearm. Have you asked the folks at Cabela's about that ammo? I'm confident they could answer your question.
If you want that rifle, I'd say get it. Once you familiarize yourself with the Sharps and the 45-90 cartridge I don't believe you'll be disappointed. I will say that the -90 is a bit heavy for target work, altho that's what it was originally designed for, due to the accumulative effects of recoil. However, it is a GRAND hunting cartridge!!!
Vic