It's not impossible, just difficult. You just have to be dedicated and persistent in your search. T-355 is a good start point, so you could be already several months late for May-July 2013, although release dates seem to have become a little more haphazard in the last year or two so I wouldn't give up. If thy do come up they will be snapped up fast so you have to on the ball; checking at, say 6pm, isn't likely to be successful, but early morning may give you more chance.