They get them from the wild and extremely large fish breeders. But besides that, I guess if you wanted to give it a shot, lower the pH in your tank by adding peat moss to the filter, of by adding bog wood and make sure you do this all in a separate tank with out any other fish in it. make sure its fairly big to. also do about 10-20% daily water changes.
These are just general things though, I'm sure there someone here who knows more about breeding glass catfish.