There are very few options available, because it is an X3.

The X3 was based on the E46 in general, but the E46xi used a New Venture 124 transfer case, which was used by other manufacturers as well. The X5 used a NV125 transfer case, same as the NV124 but with a chain instead of gear drive, since the vehicle didn't have the same tight space constraints. There are parts available for the NV transfer cases, as they are used in many other vehicles. Magna bought NV from Daimler and GM some years back, but there are aftermarket parts sources.

The problem with the X3 (and the 2004+ X5) is that they use a different transfer case, with X Drive (variable torque, not fixed). The X3 was the first model with this system, and uses an ATC400 (BMW designation). The E53 X5 uses an ATC500, presumably with more torque capacity. The E70 X5 uses an ATC700, with the same torque capacity but some minor changes.

These transfer cases may be built by BMW, or Magna, or someone else, it isn't clear. Although Magna assembled the X3 under contract to BMW, they didn't build many parts for them. It is BMW that isn't distributing parts in this case, not Magna. Since the transfer case appears to be proprietary, there are unlikely to be new aftermarket parts sources.

I did some searching for North American sources for ATC400 parts, and came up with nothing. I think your options are a used box from a salvage yard, or a new box from BMW (close to $3000 by the time you get it there). If it was just the bearing, you could try and get a bearing number to cross-reference, but the photo appears to show wear beyond the bearing itself.

After you do get it repaired, I suggest you follow BMW recommendations for the correct lube. Transfer case failures do not appear to be common. Since it is an X Drive transfer case, it uses special oil, which is rather expensive. I am not sure if that contributed to the failure, but it is worth watching out for.

Good luck.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue