Calling EAA would likely be your best bet. Despite the poor customer support, they are pretty good at selling parts, although often a maker with good customer support will replace broken parts for free whenever they should break.
EMF Fire-dragon used to make Witness barrels, don't know if they still do, but their reputation is spotty.
You may also need "large frame" .40S&W magazines with the spacer, as the shorter rounds in the longer mags often have feed issues, although sometimes you get lucky and what you have works.