Someone recently asked this but I too am looking for the AT-ST bonus parts only from Leia as well as a full R-3PO + bonus parts. Is anyone selling either of these?
That will be very hard to find. I have seen a few Hoth Leia's on Ebay, but never with the AT-ST part. These were short packed, and each box of 12 only has enough pieces to make one AT-ST (with 6 leftover parts). You might be better off just trying to buy a spare AT-ST to get your part.