Check for tattoo ink mixers. I got a cheap one I've seen rebranded all over the interwebs for a good price.
They're rebranded and available all over. I'm sure ultimatly they're not as durable as a laboratory quality vortex mixer, but I've had mine for over a year and use it daily and it still works just fine.
Drop a couple agitators of some sort in the pot, then go to town. I like vortex way more than the jiggle shakers for nail polish.