Go to http://mmpsabots.com/ and take a look at their sabot selection. Some are for .40 caliber bullets in .45 rifles, some .40 in .50 rifles, and so on. The orange colored sabot near the page bottom is for .457 or .458 bullets in .50 cal rifles.

Harvester sabots at http://www.harvesterbullets.com/sabots.htm. MMP and Harvester are the major suppliers but there are a few others.

And I don't really like any of them.