Hi Stephen,
The rotor and the mainspring (the actual mainspring: barrel complete) are available from many places including ebay and cousinsUK (if I'm not mistaken). Remember that the ETA 2824 belongs to the same family as the 2801, so those same parts for the 2801, 2836, 2804, 2840, 2842 and more are OK to use. Also, a generic mainspring from CousinsUK can be had but in my mind, it is more cost efficient to get the barrel complete. I don't know offhand the numbers but a quick search in cousinsUK will prove successful. Hope this helps.
Cheers,
Bob