Amazon.com: Line Mixer 4 Channel, Donner Low-Noise Mini Audio Mixer DC 12V with AC adapter & Stereo/Mono Adjustment, As Microphones, Guitars, Keyboards or Stage Sub Mixer, Ideal for Small Clubs or Bars :
Alesis MultiMix 8 Line
Amazon.com: LZSIG Mini Audio Mixer,Stereo Line Mixer for Sub-Mixing,Ultra Low-Noise,4-Channel,Microphone Independent Control, 1/4" & 1/8" TRS Output and Input, for Guitars,Bass,Keyboards or Stage Mixer Extension : Musical Instruments