If you needed maccha desperately right now, I'd say buy some of the 2011 harvest.
BUT, if you wanted to get the 2012 harvest of maccha, my guess is that you would have to wait until autumn to be able to get it. The only reasons why I can think this would be the case is that the process of picking, processing and grinding the 'tencha' would be very time-consuming.
Secondly, that time of year also coincides with tea ceremony of 'kuchikiri', which is the most important tea ceremony held by tea people. Before the first harvest of tea is picked, the tea masters will leave a 'chatsubo' (tea jar) at the house of the 'chashi' (the people who produce the tea).
Once the maccha has been produced later in the year, the 'chashi' will place several kinds of maccha in small bags, including tencha leaves, packed into the tea jar, ready to be given back to the tea ceremony people.
I hope that helps.