There is a voice system, the ChatterBox, which Marylen Costumes seems to recommend (or it's at least linked from the Marylen web site). According to the web site, the system weighs 5.8 lbs (2.72 kg), can easily fit under a suit, and can alter a voice to the extent that the speaking performer always sounds like the character s/he portrays, and no one observing the performance can distinguish the gender of the performer.

[Maintainer's note: Chatterbox is gone, no one knows where they went.]

Spencer's Gifts may have voice changers, depending on location. Some clerks may say that they do not carry it. Another good place to look for this would be your local costume shops, Radio Shack, and other electronics houses.