CQRobot -Speaker 8 Ohm 3 Watt for Arduino, 2.54mm Dupont interface, it is ideal for a variety of small electronic projects. : Amazon.de: Electronics & Photo
Amazon.com: Gikfun 4Ohm 40mm Diameter 3W Full Range Audio Speaker Stereo Woofer Loudspeaker for Arduino (Pack of 2pcs) EK1794 : Electronics
Amazon.com: LyxPro 40 Watt Electric Guitar Amplifier | Combo Solid State Studio Amp with 8” 4-Ohm Speaker, Custom EQ Controls, Drive, Delay, ¼” Passive/Active/Microphone Inputs, Aux in & Headphone Jack - Blue :