I'm gonna go out on a limb and say that the Hybrids haven't been altered in order to use the caster racing diffs, in which case the Hybrids would still work with other brands like ofna & hotbodies ( and LST diff internals too ). In that case you could start from scratch with new internals which are easier to find parts for ( never known someone make such a meal out of assembling diffs...

), or you can go to ebay & type in 'caster racing'- you will find various bits of diff including the ring, pinion & spider gears. Trouble is they sell various different models with different didds, so that complicates things.
You may be able to look up part numbers on the caster website, though you won't get any joy when it comes to asking about compatibility; FLM could have gone with any brand of diffs to fit their hybrid bulks, but that brand has no input or affililation with FLM. I'd just try and find out exactly which model(s) the diffs are for, that will make it easier to find the right spares.