I have an '03 and I was having intermittent problems getting the tranny into reverse. The dealer replaced the transmission under warranty. The short answer to your question is yes, they are rebuildable. The dealership service departments find it more expedient to swap the tranny out and let someone else rebuild it rather than spending time on diagnosis and repair themselves.
One place I found online that does transmissions for Coopers is Mini Transmission.com (
http://www.minitransmission.com). I know nothing about this business, nor have I talked to anyone who has used them.