Howdy
You are seeing nothing but 20 round boxes of 45 Colt these days because of the current shortage of ammunition. I just checked Midway USA and almost all of the standard 45 Colt loads they are listing are out of stock. This is partially due to the pandemic, when so many businesses had to shut down, and the ammo companies have not caught up yet. It is also due to panic buying and hoarding. This happens every few years.
45 Colt is a great cartridge, it has been around since the introduction of the Colt Single Action Army in 1873.
There is no question 45 Colt is more expensive to shoot than 38 Special or 357 Magnum. First off, the bullets are bigger and heavier, so more lead means higher cost. I discovered a long time ago the only cost effective way to shoot 45 Colt was to load my own cartridges. That is still true today, it is much less expensive to load your own instead of buying factory ammunition. Unfortunately you are coming to it at a bad time. Just like the ammunition shortage, there is a shortage of loading components too. Partially because whenever there is an ammunition shortage, more shooters start loading their own, and that leads to shortages. I buy most of my brass from Starline, and they are currently out of 45 Colt brass. I have never seen that before, they have always had it in stock whenever I needed any. Perhaps Starline will be able to meet the increased demand soon. Reloading Dies are hard to come by now too, as are primers. When you are able to find a set of 45 Colt dies, be sure you spend the extra few dollars on carbide dies. With ordinary steel dies you have to lube the cases or they will get stuck in the sizing die. A carbide set of dies has a ring of carbide in the base of the sizing die. The carbide surface is so smooth no case lube is needed. Hornady does not make carbide dies, but their dies have a Titanium Nitride coating which allows cases to be sized without needing case lube.
I started loading 45 Colt about 20 years ago when I first started shooting in Cowboy Action Shooting. One of the beauties of loading 45 Colt is the components are large and easy to handle. Do yourself a favor and buy a reloading manual, and read it cover to cover. Don't rely on you tube videos to show you how to load cartridges.
I have been loading 45 Colt and several other cartridges with Black Powder for many years now. The amount of Black Powder you pour into a cartridge actually varies slightly because not all Black Powder weighs the same. My rule of thumb is to pour in enough powder that when the bullet is seated, it will compress the powder by between 1/16" - 1/8". My standard load these days is 2.2CC (about 33.3 grains) of Schuetzen FFg under a 250 grain Big Lube PRS bullet. I usually use Federal Large Pistol primers, but any Large Pistol primer will work fine. Don't listen to guys who tell you that you need Magnum primers for Black Powder, it is not true. Black Powder is actually easier to ignite than Smokeless powder.
I don't actually scoop out powder with the Lyman dipper in the above photo, it is just there for reference. I load all my cartridges on a Hornady Lock & Load AP progressive press. This is an old photo, there are several different brands of brass on the shell plate.
When loading Black Powder into cartridges you need to use bullets that have a Black Powder compatible bullet lube on them. The bullet lube on regular cast bullets usually does not mix well with Black Powder. It tends to combine with the fouling and leave a hard, crusty fouling in the rifling that ruins accuracy and is difficult to remove. There are lots of recipes for Black Powder compatible bullet lubes out there, the lube you see in the above photo is SPG, bullet lube. Generally speaking, a BP compatible bullet lube will be soft and gooey. The bullet you are looking at is the Big Lube 250 grain PRS bullet. The Big Lube series of bullets have a huge lube groove that carries enough lube to keep the barrel of a rifle coated with soft lube its entire length, doing away with the need to swab the barrel to keep up the accuracy. This photo shows a 44-40 cartridge on the left and a 45 Colt on the right. Beside each cartridge are the Big Lube bullets I use in them. I have removed the lube from one of each bullet to illustrate how large the lube groove is.
I have very little experience with the Black Powder substitutes. I will caution you to stay away from Pyrodex, it is said to leave fouling behind that is difficult to clean up. I do know that American Pioneer Powder does not need special BP bullet lube, it works fine with modern lubes.
Just to head off any misunderstandings, despite its name, Trailboss is not a Black Powder substitute. It is a Smokeless powder and never should be used as a substitute for Black Powder.
One other thing, despite what you may have heard, Black Powder is not as corrosive as many shooters think. I have often gone over a week before cleaning my guns after shooting them with Black Powder. I am not going to admit in public how long, but they did not revert to being piles of rust. Cleaning up after shooting Black Powder is messy, much messier than cleaning up after Smokeless, but with a good water based BP solvent it is easier than cleaning up after smokeless.