[I've got loads of these.](https://www.banggood.com/100pcs-5mmx2mm-N52-Strong-Round-Magnets-Rare-Earth-NdFeB-Neodymium-Magnets-p-988439.html?rmmds=myorder) I wouldn't say they're super strong but they can hold a pencil to a fridge --- will probably work for a hall effect sensor. Send me a PM and I can try get one to you.