I was in the same boat till I called Moryde a few days ago, you can't find the part number online presumably because it's a part of a larger kit.
RPB710-009 is actually one of four nylon stability blocks that come with their RPB77-006 Gooseneck Adapter kit that goes for a little more than $200.
Or you can buy replacement parts individually for a little less:
RPB710-009 (stability pad): $28.99 each
UO115-461 (bolt) : $1.09 each
UO117-109 (washer) : $0.29 each
Call them at 574-293-1581 and ask for parts, your only other alternative is to find them on eBay, etc, I found two of the stability blocks there, or buy the whole gooseneck kit.
Per some of the other threads, I don't think Andersen is saying you need the Gooseneck skid plate in the kit, you would actually have to weld a gooseneck tube (or hitch pin) to it anyway. You just need a few parts from the kit, the stability pads (4x) and bolts (4x) to lock out lateral movement since the hitch is not sitting on the fifth wheel skid plate, I suspect the pin has a tendency to lean side to side since the Andersen hitch is sitting on ball vs a skid plate and probably puts some undue lateral forces on the rubber isolation block in the Moryde rubber pin box, presumably increasing the risk of damage and/or premature failure. I haven't installed them yet, but I believe they will allow forward/aft movement while locking out side to side movement. I wish Andersen and/or Moryde would do a better job of explaining this on their website(s).
Hope this helps!