Shimano front-derailleur with 28.6 Clamp
You can buy a braze on derailleur of the type you need and get a Braze on to 28.6 adapter. Alternately, you can get a derailleur with a larger size and get a shim. Either will be perfectly functional. Very few Mountain Bikes have 28.6 downtubes anymore so the likelihood of finding a 28.6 seems pretty low.