The species you are misting in part determines what type of misting system you can build. This is very often overlooked, in my opinion.
For species that drink out of a water bowl one can probably get away with an ultrasonic humidifier. These do an excellent job of raising humidity with very little water. The only downfall is that you really need to use distilled or RO/DI water in order to not ruin the unit.
For species that need enough of a mist to leave water droplets on cage walls, furnishings, etc., then a high-pressure pump in preferable. These can be more difficult to find parts for.
Lastly, some arboreal snakes benefit most from a fairly large water droplet size in order to induce defication. These can be built easily with commonly found parts.
Other important considerations are wether or not you want the system to run on a timer or if you can live with someting you turn on yourself.
One cheap system to consider if you don't need a timer is that availabe from Doug Bar (herpcages.com). Great guy, great product. You just hook it up to a normal old pump-up garden sprayer. Probably not the ideal system for large water droplet sizes, though.