Jumping out of 3rd is usually a synchronizer, input bearing or pilot bearing issue.
That's the part number for the entire tranny. You won't find a new one, or likely even a rebuilt one with that number. You can find part # information here:
29-54 Parts ManualSome of the vendors carry some transmission parts, although gears are very hard to find, and most bearings can be purchased locally with the PN on the original bearing.
