I would like to add a small strong magnet to my motor-axel, to use it for measuring rotations at a higher resolution than 2/round .. but my two electro-pushers do not have a hall-effect sensor with a more continuous output. <snip>
Do you want a linear sensor or a non-contact encoder? Both are available.
Linear hall effect sensors are widely available. I entered linear hall sensor into the search box at digikey.com and got hundreds of matches. FYI, this URL should reproduce the same search results: http://www.digikey.com/product-search/en/sensors-transducers/magnetic-hall-effect-digital-switch-linear-compass-ics/1967232?stock=1
Hope this helps