I need mag extensions for an XD 9mm for USPSA. I shoot limited so the need to fit the 140mm mag guage. There isn't a lot of info on what will work and what won't.

Arrendondo extension will be legal and are very affordable.  Get the new longer magazine spring when you add the extensions.  I have an XD-40 with 6 of the Arrendondo extensions on 6 magazines.  I have broken one in the past 8 years of using them and that was after a lot of drop onto concrete shooting indoor matches.

Basepad

Springs


Pistol Gear has their own brand of 140mm extension that is a pretty good kit also.  Includes new follower and spring along with the basepad.

Pistol Gear Ext. (scroll to the bottom)

Springer Precision: XD 9/40 140mm Pad