I've heard the absolute best time to get boxes is actually late summer/early autumn. But that specifically applies to British produce which is at it's peak then. Some of the box schemes, such as Riverford here in the southwest, are cooperatives who work with both UK and international farmers. That way, during the 'boring' months, they can supplement with produce from other countries. Many of them have environmentally friendly shipping policies, too. In other words, they won't defeat the organic message by air freighting you apples from New Zealand.