Magnets are easy. You almost certainly have some laying around already. Got any old hall throttles or hall ebrake levers? At least one in each of those. Got any old namebadges with magnetic clips? Usually three on each of those. Got any old cheap disc brake calipers? Probably at least two on those, holding the brake shoes in. Got an old cycle computer? Magnet on the wheel sensor. Computer case fans? A whole strip of magnetic material inside the round core of the fan itself. Old earbud headphones? Ring magnet in those, probably. Same for old small devices with tiny speakers, like celphones, or even regular phones sometimes.
Hall sensors...many of the computer case fans have hall sensors in those, of hte type useful for ebrake switches. Old throttles will have the hall type you need for yoru custom throttle. Something else might, too, but I can't remember right now.