Oasis Case Humidifier
This case humidifier made by Oasis fits in almost any instrument case. A choice of two attachment methods (a clip or a magnet) makes it easy to use, and it's simple to tell when it needs more water because it shrivels up and looks dry. A handy syringe is included so you can refill it without removing it from the case. If your climate is dry, this is a great way to help keep your instrument happy!
Depending on the size of your instrument and the dryness of your environment, it may take more than one humidifier to do an effective job.