The slower twist works fine with sabots...When Knight introduced their first inline....It had a 1-32 twist...Some guys still use them...
Go to Wally World, Gander, Dicks and see what the local stores have....Hornady packs 20 or so in packages with the corresponding sabots....I have had good luck with the 250 Shockwaves and the 300 grain Hornady XTP....Buy a pack of each....If your gun happens to like the Hornady XTP, you can buy then in bulk...The Shockwaves....I wouldn't buy bulk until you know what your gun likes....Then you could buy bulk sabots for MMP and the .451 Hornadys and shoot cheaper.....
One other thing....I always clean the barrel between shots....Get some rubbing alcohol, put in a spray bottle, dampen (not wet) two patches and swab the barrel, use both sides, then run a dry patch down and then a lightly oiled patch, dry again....These guns get hard to load if you don't clean between shots, and many won't group worth a dime as well when dirty....
If you are using Goex, I'd start with 80-90 grains of powder and work up to the max the book advises for that gun....In my Knight I use 90 grains of Goex and can keep all shots in a inch group...If I go up to 100 grains....the dang group opens up to 1 1/2 - 2 inches....
PS....If your gun does not like the 250 Shockwaves, they also come in 300....
PPS...Don't let the 1-32 play with your mind...Muzzleloaders are pretty forgiving....I would bet you could take any good 250-300 grain bullet and get it to give 1-2 inch groups at 100 yards by just adjusting the powder charge and changing sabots....Make sure that you take notes on each load used....Its easy to forget what was best at the end of a shooting session...