Most if not all RE magnets are now made in China. Search something like "Rare Earth Magnet China Source" For Hall sensor magnets, the magnet quality should not be a problem so the lowest bid might be the selection choise.
About 75% of NdFeB magnets are manufactured and supplied from China. The five licensed companies are: BJMT (Beijing Jingci Magnetism Technology), Yunsheng, San Huan, Thinova, AM&T.
About 22% come from Japan: Shin-Etsu, Hitachi, and TDK.
About 3% come from Western Europe: Vacuumschmelze.
As suggested above, a search on the internet will find literally dozens of sellers of these magnets to companies in the USA. But you would be wise to stick with a reputable company as Neo quality and dependability of supply can vary greatly.
Another possibility is to use a reputable US magnet company to source the material for you, acting as a quality and supply screen for you.