I had done a lot of research about the conversion cylinders and came across mention of those .45 ACP conversion cylinders. As I understand it, one good option is to find someone who can reload .45 ACP as 'cowboy loads'. All the recommendations I've seen say to make sure to keep the muzzle velocity under 1,000 FPS.
Take a look at these:
A lot of these are .45 ACP rounds that are under 1,000 FPS.
Another thing I noticed is that while on the Kirst Konverter site, there's no specific mention of what kind of .45 ACP is safe to use, I noticed that there is the <1,000 FPS warning at many of the places that sell the cylinders:
Hope this helps a little.