Hi Lucy. Not sure what you mean by 'spares' - do you mean just sails? There are numerous sailmakers who will make you a new mainsail. Unlikely you would find a used one unless a Moody 34 owner happened to have an old one they didn't want.
The steering on the 34 will be a Whitlock system. Best bet for an autohelm to fit on it would probably be a Raymarine wheel pilot, I'm not sure of costs but if you look them up on a retailer site such as Ocean Chandlery, or Cactus Navigation, you should be able to find a price.
Hope this helps.