Damn, the entire internet is out of stock it seems. However:
http://www.advantagehobby.com/produc...=67666&cat=970 due 26th
and:
http://cgi.ebay.com/101036-HPI-43T-S...item27bb16cb6b
says its for the hpi trophy 3.5- it looks identical though so I'd guess its the same as the lightning buggy diff part... possibly.
One last option:
http://cgi.ebay.com/New-Ofna-Hyper-7...item33687480c8
Those will be a direct fit for the FLM hybrid bulks, or just rob the ring gear and maybe the pinion too if the teeth dont quite match up perfectly with the HB gears- they should though really. Only $5 more than the HB ring gear on it's own + you get nearly a whole spare diff...