Indeed, there are no direct busses.
You coud take a taxi. That would be the most direct way.
Using WDW transpo, you could go ASMo to MGM, then walk (@ 15 minutes) or boat (may be a bit shorter than that if a boat is at the dock, and you get off early at the S/D and walk over).
You also could bus to DTD or AK, and bounce to a BW bus from there.
(Don't bus to EPCOT, b/c no BW busses there, so you'd have to walk through Epcot, and then out the back gate).