Get them at Walmart in the section where they sell replacement battery packs for cordless phones (not cell phones, cordless handsets that have the base unit attached to a landline). They are already shrink-wrapped and you may only need to splice in a longer wire. Best to place the battery pack in the floor of the organ rather than mounted on the pc board, for safety in case it leaks.
You'll need a 2-cell AA pack for the memory board, but also replace the 3-cell pack on the USAV board in the cage. You have to pull the card out of the cage to do this, and again, don't put the new pack on the pc board. Add a long pair of wires and put the batteries in the floor. Bend up a corner of the cage cover to allow the wire to exit without being pinched.
You may find the battery pack has leaked on the USAV board, and if so, look for a thread on this forum all about repairing the cage and the boards when this happens.