Anong brand ang gusto mo? Kung generic lang, any electronics supply store have one. But you need to know the diameter of the speaker that would fit the location and the resistance (ohm) and power (watts) requirements ng HU mo - you could get this info sa manual ng HU mo - dun sa specs sheet.

Kung branded, you need to contact our audio gurus like glenn o kaya visit KAC site for better opinions/suggestions