I am sure there are knock offs and diy ones available as well.
Basically, the trick is to not knock the CO2 out of suspension when transferring. To do this, you need to maintain pressure in the bottle while filling.
I believe you can do this with a drilled stopper and a racking cane. Just tilt the stopper a bit to let the gas escape as the beer enters.
I use the drilled stopper, plastic tubing, and flip top bottles approach and it works just fine, but I only transfer to bottles to share with friends. So, the brew is typically not in the bottle for long. I suspect that if you wanted to bottle for competition etc where the brew would be in the bottle longer then maybe a counter-pressure filler like the beer gun would be better.