First off, the 1872 Colt is not a conversion. It was a production open top cartridge revolver. It was not successful since the Army said they would not adopt an open top design, so Colt designed the 1873 SAA.
Now for the ammo. Every company out there makes some cast bullet .45 Colt loads. Any of those will be fine in the 1872. I reload for mine and have never put a factory round through it. But if I had to buy some factory loads for it, any standard pressure cast bullet load will be fine.
Another option out there is the replicas of the Richard Mason Conversion of the 1860 Army. I also have one of those, but that one is in .38 Special. It is a fun, soft shooting and accurate revolver.
The one big bonus of these over the SAA replicas is compensating for windage. With a SAA replica, you have to put the barrel in a vice and turn the barrel in the frame to lean the front sight one way or another to adjust the windage. You can pretty much plan on doing this if you get a SAA. Out of five SAA replicas that I own, only one came from the factory adjusted correctly windage wise.
Neither of my open tops were sighted in from the factory correctly for windage. The factory plans for this by leaving the rear sight extremely narrow, almost unusable. When you put it on paper, just use a small triangular file to enlarge the rear sight notch in the direction you need the bullet impact to move. It works equally well with the rear sight notch in the Richard Mason's hammer or the rear sight which is part of the 1872's barrel.
Basically, if you want a single action to use heavy loads or JHP defensive loads, buy a Ruger. If you will be happy with black powder equivalent cast bullet loads, pick one up. They are a lot of fun. I love mine.