The ODST File set in the Armory has them for Pep and Foam, the yare the "Hip Plates."
This is a comprehensive zip file including numerous files from multiple artists. Contents of zip-file: halo-3-odst-unsc-odst-belt.pdo Modeler: Nightshade halo-3-odst-unsc-odst-bicep.pdo Modeler: Hugh Holder halo-3-odst-unsc-odst-boot.pdo...
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Unless you are looking for an additional piece that goes on top of the Hip Plates that the Grenades attach to? In which case, no such piece exists as the grenades are just sort of stuck to the hip plates in Game with no actual visible mounting or attachment system or item.