I got it from the same guy here.
http://cgi.ebay.com/SENSE-G3-T7-Vacu...item3f06bd8537

Be sure to contact the seller about the power supply's voltage.
But there are tons of other OTL amps such as Little Dots, EarMax, WooAudio, Darkvoice, Antique SoundLabs, etc.

How much did you get the DT770 for?
The 600 ohm version mates best with OTL tube headphone amps in my opinion.

JRA