As Paatz says, reloading is your only practical option here. Even if you find a box, it is likely to be $$$$. It isn't difficult to get into reloading for this, just a little pricey on the upfront cost for the bra$$ but frankly the brass lasts forever if you do your part (neck size only, anneal as needed, etc). Bullets are the same .458" is .45-70, .458 WM etc. You can reload .45-90 with .45-70 dies if stuck but dies for the '90s are available and no difference in cost (depending on where you find them anyway).
You can also shoot .45-70 out of a .45-90, although depending on the throat of the rifle in question the accuracy can suffer. Unless you reload with the bullet seated out far enough to get into the throat (or at least reduce the "jump"), then the bullet has to "jump" more than it does in the usual .38/.357 chamber analogy, so it isn't the same thing. If you try that:
1) DON'T use the modern "Leverution" (sp?) or anything hotter than "Trapdoor pressure". Again, reloading is the best bet here...personally I'd avoid factory modern ammo...YMMV
2) If it is a 19th century bore, I'd personally avoid jacketed bullets and stick to cast although that is only one opinion, seems to be evidence on both sides there.
I would suggest just setting up to reload the .45-90 as-is, it is a great hobby and even if your dad decides to sell the rifle, it can be sold a little easier if the components to shoot it are there with it. Good luck in getting the old girl shooting, lotta folks here with a lot of knowledge in doing exactly that if you're stuck.
Edit: I should also mention, my personal preference on this caliber in an original, is BLACK powder; but you can certainly, safely load it with smokeless just do your homework; lots of info on this forum and on the intermanet in general on that topic.