Hi, Had to do mine last year. Its easy if you have some mechanical knoweledge. You have to drop down the exhaust and remove the prop shaft, few wires and bolts and it will come out. You can get the torque converter off with the auto box by unbolting the converter from the flywheel from the front.
If you look on ebay there are several BMW workshop manuals on CD's that have been copied from dealer versions. There not much money, but very useful, it explains how to take the box off.
Many gearboxes suffered this fault, code 5HP24, so make sure you know the new box is good. Also see if you can get the gearbox ECU for the box as some people say that they are paired up (mine worked fine with the standard ECU).
Cant really advise on where to get the box from, sorry,found mine on ebay.
Hope this helps!