For the finest spray from the cheapest bottle, I'd look into a surprising source... the girly-products aisle of the supermarket!!!

The finest re-fillable mister I've found comes on a product made by Neutrogena to treat dandruff overnight- it's their salycilic acid anti-inflammatory spray ("Neutrogena T-Gel Overnight Dandruff Treatment") and has a weird-lookin' nozzle on it that supposedly gives a VERY fine mist for a non-aerosol product. Hang on let me see if I can find a pic of the shit. There. Lots of specialty hair products come with very high-quality, low-droplet-size misters. I'm not saying they are as good as an ultrasonic mister but certainly better than a regular spray bottle for small applications.