Hmm it would depend on what is flogged out in your diff I imagine. The guys at bmw aren't going to spend time overhauling a diff as they mentioned they don't sell individual components (which i find hard to believe)
Im happy to work together with you & try and source you the components you need to get your diff operational if it means you will purchase my wavetrac diff centre
I must admit I'm not a diff expert but if your ring gear is ok I know the wavetrac diff centre bolts up to the oem ring gear....
Alternatively you could call a few wreckers, but whenever I have called them for parts they want ridiculous prices and you generally dont want a diff from a wrecked vehicle if its been involved in a shunt from behind.
Unfortunately wrecked 335i's are also quite rare in Australia, and you need the 335i specific diff as a 328i etc have different ratios.
Honestly if I was in your shoes, visit a reputable diff specialist have chat to them and see how much they will charge you to diagnose the issue by stripping it down, and then once you know exactly whats wrong work out what components you require and order them in.
May I ask how many kms have you done, and have you driven it hard?