Based on this product's specifications, here are some alternatives and similar models
Headphone Amplifier Terratec HA-1
Summary description:
The Terratec HA-1 headphone amplifier delivers peak power of 0.06 W per channel with a high signal-to-noise ratio of 95 dB. It supports headphones with an impedance of 16 - 300 Ω and features a 3.5mm connectivity. With a USB power source and an impressive battery life of 150 hours, this compact amplifier (53 mm wide, 10.7 mm deep, 111.5 mm high) is perfect for audiophiles on the go.