The standard sabot bullet for the .45cal rifles when they came out a few years ago was .357 bullets in the sabot. Unfortunately they were all way too light to get accuracy from if the velocity was pushed by the "magnum" loads the manufacturers were advertising.
Your best bet would be to try the 180 .357 bullets with a powder load of around 80 to 90gr. Anything much hotter will ruin accuracy more than likely.
Sabot quality CAN be an issue like articap says - get ahold of some MMP sabots and you take that problem out of the equation.