You've checked etrailer, ebay, and Lippert/PG itself, right? Lippert seems to list it:
https://store.lci1.com/motor-with-7875-drive-shafvt00310r-ven075-vt00310.html
RVUpgrade Store has an extensive listing for slideout motors - maybe you can identify a matching one there:
https://www.rvupgradestore.com/Lippert-Motors-and-Actuators-s/286.htm
Vt00310 is a Power Gear identifier, so you need to find a cross-reference to the actual motor manufacturer's part number in order to locate a source other than PG/Lippert. Might find it hidden under a cover.