Reproduction and OEM parts are available through Midwest Military, Brent Mullins, and Ron Fitzpatrick (
www.g503.com). DL Bensinger has reproduction parts, as do a few other vendors. The reproduction parts are most likely from MD Juan in the Phillipines regardless of the vendor. Not inexpensive, but not unreasonable when weighed against the convenience factor.
You can sometimes find someone parting out a M416 trailer on various message boards such as here, g503.com, the M416 Yahoo group, "pirate," "Ih8te Mud," etc. Prices are generally less, but condition varies and you need to be patient and wait for what you need to become available.