Are filled - that is, ready to be handloaded - CCI 45 Colt shotshell capsules available for purchase? If they are, please identify from where or from whom. If they are not available as filled capsules, are there similarly filled, ready-to-be-handloaded .45-caliber shotshell capsules available from other sources?

You can buy the unfilled plastic shot shell capsules from Midway USA and from other places as well.
Tnen you fill tnem with lead shot and load them your self. You must be careful to get the crimp right so you do not break tne plastic shot cup, but xtill have enough of a crimp to hold the shot cup in place under
recoil.
I first loaded the Speer shot cups myself in the mid 1970 in the 44 Special/Mag. Back then then shot cups were yellow.

For the most part, since CCI Speer started making factory shotshells I have just bought and used them.

The factory CCI/Speer shotshells are EXCELLENT. I have used hundreds of them in 22LR, 22MAG, 9MM, 38 Special/357MAG, 44 Special/44MAG, 45ACP, and 45 Colt.

The only time I still load my own is when I have need for something special, like 45 Auto Rim shotshells or in the 44 Mag/Special, and 45 Colt a larger shot size, like 7.5 or No 6.

I have killed a lot of snakes and s all game for the pot with CCI/Speer shot shells.
I really like them.

I've used the factory loaded rounds in 357mag from a 94 Trapper a number of times, and yes,they are quite good.
Although one time, we were firing patterns with them onto an old refrigerator we had set up at about 20 yards, when one of the capsules failed to break up and the whole slug assembly (visible in flight) went right through the side of the refrigerator in one piece, just like a bullet.
Be a little careful with those things, they may not break up every time and will travel a long way indeed if they remain intact.