Maker (Rex) bullets have a proven track record, you can buy the bullets for reloading yourself, or there are vendors that use them in subsonic ammunition (Mid America Munitions, for example, but there are others).

As stated above, the Hornady Sub-X have proven effective!

Lehigh Defense also have a great track record: you can buy their bullets to reload, or there are vendors that use them, such as Gorilla ammunition...