I stir paint in bottles, tins, pots with opening lids, etc., but I shake eyedropper bottles.

Electric paint shakers aimed at modellers are greatly overpriced compared to the exact same devices intended for different audiences, BTW. I don't recall who else they're marketed to, but I do remember some years ago on another forum where somebody came across one for around half the price of those sold for shaking eyedropper paint bottles. At the other end of the scale are laboratory shakers, which are much more expensive when new but can apparently be had cheap second-hand. Oh, and you can also make your own from a woodworker's glue clamp and an electric jigsaw