What you need is called a talk box.  It's often referred to as a talkbox, vocoder, or voice synth.  The best brand out there is Banshee Rocktron Talk box.  It'll run you about $150, but it is totally worth it.  It is a lot like a stomp box or distortion pedal for a guitar, but you can use these for any instrument or just your voice.

They are easy to hook up.  All you need is the talkbox, the power cord, and your input/output(such as mic, amp, guitar, etc.)   What you do is plug it in, plug the amp in on one side, and your guitar or mic on the other side.  You're good to go!

Rocktron Banshee Talkbox for guitar    US $49.00

Rocktron Banshee 2 Talk Box Guitar Effects Pedal New!    US $189.00

Rocktron Banshee Talkbox    US $10.50

Rocktron Banshee 2 Talkbox, Excellent Condition    US $175.00

Rocktron Banshee 2 Amplified Talkbox NEW IN BOX!    US $200.00

Rocktron Banshee Talk Box great condition    US $1.00

Rocktron Banshee 2 Talk Box - Amplified - Brand New!!!    US $249.00

Rocktron Banshee Talk Box Guitar Effects Pedal - NEW!    US $139.90

Rocktron Talk Box Banshee 2    US $170.00

ROCKTRON BANSHEE AMPLIFIED TALKBOX    US $130.00

Rocktron Banshee 2 Talkbox    US $100.00

Rocktron Banshee Amplified Talkbox 1997 print Ad    US $9.99

Rocktron Banshee Talk Box    US $169.00

Rocktron Banshee Talk Box Guitar Stompbox BRAND NEW    US $134.99

Rocktron Banshee Talk Box FX Series 1    US $159.99

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