HOG Hunter. Before you do anything!!!,,,, you have that gun checked out by someone who actually knows what he is looking at and talking about. The gun is 130 yrs old. I can't stress this enough. These are not strong guns to begin with, and if it isn't right you could easily end up eating it! literally. Get it looked at by a competant gunsmith first.
That being said, You can buy brass from Midway USA and I would recommend the Starline Brand, however any of the name brands are good for the low pressure rounds you'll be loading for this gun.
I would recommend the Lyman 49th edition reloading manual (the current one) which has loading data for "all power levels" of .45-70 from TrapDoor, to Ruger #1.
***Do be wary as you will only be loading that round to BELOW 21,000 PSI.***
There is much data available for loading that round. Don't get confused and try to feed it ammo meant for a Marlin 1895 or Winchester 1886, It won't take those pressures.
Also I would look at the Hodgdon Annual Manual(magazine version) as it has data for it's powders like Trailboss which you can't hardly hurt yourself with.
As far as Bullets go, you will be using cast lead bullets. Period. There are a bunch of outfits that make good cast bullets. I would suggest looking a Beartooth Bullets, Oregon Trail, and Cast Performance among others. All have bullets made especially for the rounds you'll be loading.
If you are going to cast your own bullets (recommended) then Midway USA sells just about every bullet mould known to man, as well as everthing you need equipment wise.
The two bullets the round was origianlly loaded with were 405gr and 500gr. You won't like shooting the 500's with a steel buttplate. I personally shoot 300 gr bullets mostly in my Marlin. Even at 1300fps they pack a serious wallop on the receiving end but are pretty painless on the shoulder. Any "plainbase bullet" between 300 and 430 gr will work well. Just remeber the bigger the bullet the harder the kick.
There is literally an endless supply of information on the internet about everything concerning this cartridge, from guns to loading for everything from plinking to Elephants. It was the first American Military self contained cartridge 138 years ago. It has taken every kind of game on this planet as well as some of the larger passenger cars and some light trucks. It is more popular now than it has ever been and is only getting more so everyday.
It is one of the good ones, and you won't be satisfied just having one rifle to shoot. Next you'll be looking for an 1895 Marlin or a Winchester or Browning 1886, so you can shoot heavyer rounds. It is a blast ,,,,Literally.
It is a fun cartridge to use and can be just as serious as you want it to be.
Good luck, Hope this info points you in the right direction.
Alliant Reloder 7 or Hodgdon H4198 works well. I prefer magnum primers. Trapdoor level loads have plenty of oomph for about any use, or game.
Berry Bullets makes a great plated bullet. This bullet will not hurt your barrel. There are also plenty of cast bullets available for this grand old warhorse.
I have an H&R Buffalo Classic. Although the factory says it will take anything a Ruger no 1 will take (OUCH), I load only to trapdoor levels. It is all that is needed. It is one of my all time favorite rifles to shoot offhand.