I've never seen a Duo either. It seems like anything PCE, TG, or Duo is super rare, even in a prime retro market like Portland. We have the biggest retro show in the country and several smaller shows each year, yet the TG stuff remains elusive. At any show, you'd see several Vectrex, Virtual Boy, and Jaguars for sale, but very rarely a TG-16. I know a guy that found and bought at TG-16 at PRGE two years ago, and it ended up not working.
The good news is that the everdrive appears to play the Japanese titles on a TG-16, so I'm really looking forward to that.