Stimming is a great deal of fun. I need to replace my power box which finally gave out after many years of use. Next time I want an ET-232 or an ET-312. Just do some reading and Fowfer has given you a great resource for some beginning information. Also remember that some individuals need some specific kinds of gels or other conductive lubricants with some of the lower powered units and in addition a large quantity of body hair in your contact areas can greatly diminish your stimming experience.
This is a great deal of fun,
Good Luck!