You may get lucky and someone here (dealer or otherwise) may have a panel sitting on a shelf, or have a complete unit they are parting out. Short of that, you may need to source one via eBay or similar, possibly in better condition overall and simply replace yours (keep it for spares). One advantage you have is that the BC4500 (I believe a 3500 panel will work also) is the least desirable unit of this system and you should find one for relatively little cash outlay.
I would recommend rebonding all of the remaining panels at this time (including the new one you get). Eventually, they will all hit the ground if not addressed!