I did some research on shooting the 357/45 combination.
It looks like MMP is the only one currently making a 357/45 Sabot in the Blue Version. The Orange "Longer Petal" MMP sold by Precision Rifle is no longer listed on the web site.

Per my reading/research the 40/45 Sabot is a much better option. It seems that the extra thickness of the Sabot when shooting 355-357 bullets can have issues especially at the higher velocities. It seems Sabots have problems when too Thick or too Thin. It seems they have a sweet spot in the thickness between the Bullet and Barrel.

Shooting 40/45 Sabots you will need to pay close attention to bullets. You will be able to wind them up well past 40 cal handgun velocity in the muzzleloader.

The best thing would be for MGM to get a 50 Cal barrel in production I would think.

Bob