You have a number of options depending on the size and depth of your stones and the area to be cleaned:
If the area to be cleaned is small, you could manually do it with a pick and shovel. If the stones are really big you may need a jackhammer.
But it sounds like you have a commercial size orchard, so I'll give you the options for large machine clearing.
If they are reasonably small and shallow, you could use something like a beach cleaner:
There are small ones like above.
And there are large, tractor pulled varities
If the stones are large, you may need a backhoe
You're best option is to grab the whitepages and find either a rental company in your area or a contractor to do it for you. They will have the right equipment and advice for an undertaking of this size.