Sorry, can't comment on "commercially available" harnesses. MrsSpectrum (DW) is pretty handy with her sewing machine, and--if asked--would relish the challenge of something like that. She might even opt to crochet one.
That said, I've been mulling over outside walks for some time. Some sources caution that there could be (depending on individual circumstances) exposure to lawn & garden pesticides/herbicides, parasites (if s/he eats any bugs or plants) or impaction from ingesting soil, pebbles, etc. Also keep an eye out (depending on your local circumstances) for predators (cats, dogs, birds, other wildlife). A harness might prevent your dragon from being spirited away, but damage could still be done.
Food for thought.
Honest, I'm not against this--just think & be vigilant. The whole idea of a harness is to protect the l'il buggers from harm, right? That's only part of the story when they're out in the big dangerous real world. :wink:
YMMV