Where can I get a barrel for benjamin discovery

Some choices would be a .25 marauder barrel. They are 1/2" or .500 as compared to the 7/16" disco barrel. They are also shorter than the disco. I've read where it's not advisable to turn down the end of the choked barrels, and with the dimensions of the BNM breech your limited to about a 1" OD max to clear the air tube. I built one gun and used a .470 177 cal LW barrel with a cutdown BNM shroud. Two screws secure the shroud as compared to the threading on the marauders. You can get LW barrels from their supplier in Georgia. The money you will spend , marauder trigger, and barrel plus machining, a used or refurb .25 might be a better alternative.