There are lots of software ones. Some are free, some are cheap.
Hardware wise, as suggested already you can check out Boss SE series processors. I'm not sure about an SE-50 but the SE-70 had a great sounding Vocoder. A couple of different types if I recall correctly.
You stil need a synth with those. Because in essence they let you vocode one input against another.
If you can afford something a little more up to date check out the Korg Microkorg. Basically a VA synth with built in Vocoder and a mic. You can do stuff like freeze the sound's formant and split the bands over the stereo field.
But if all you need is stuff like robot voices or cascade type sounds an SE-70 will do.