Monoprice via eBay has Monolith Liquid Spark Headphone Amplifier by Alex Cavalli (33304) on sale for $32.49. Shipping is free.

Alternatively, you can buy more for an additional discount:

• Qty 2 $61.72 ($30.87 each)
• Qty 3 $91.62 ($30.54 each)
• Qty 4 $120.84 ($30.22 each)

Monoprice via Target also has Monolith Liquid Spark Headphone Amplifier by Alex Cavalli (33304) on sale for $32.49. Shipping is free w/ redcard or on orders $35+.

Specs:

• Dimensions: 4.6″ x 3.7″ x 1.5″ (117 x 94 x 38 mm)
• Weight: 9.6 oz. (271g)
• Headphone Output: Unbalanced stereo 1/4″ TRS jack
• Preamp Output: Unbalanced stereo RCA jacks
• Signal-to-Noise Ratio: 108dB (ref. 1 Vrms input / 2 Vrms output @ 1kHz)
• Crosstalk: –73dB (ref. 1 Vrms input / 2 Vrms output @ 10kHz)
• Output Impedance: < 0.1 ohms
• Gain: +3dB or +6dB
• Input: Unbalanced stereo RCA
• Output Protection: Delay and offset protection