I had trouble with this when I needed to book tickets to the states. Even though I wasn't able to book them until a couple of weeks before I travelled (had to wait for the visa and confirmation of accomodation), I still couldn't buy the return ticket so far in advance (outbound was mid-August, return was mid-June the next year).
What I ended up having to do was buy a return ticket, coming back in December and then a another return ticket (to the UK and back to the US), only using the outbound parts of each.
It was a little complicated because I booked the first flights in case I wished to fly home for Christmas, but when I didn't go home, I had to sort out the tickets for June. The cost didn't really matter that much, because the government (my LEA) paid both fares for me.
The one thing that you may find is that 2 return tickets can actually be cheaper than 2 one way tickets. I will be flying to Albuquerque, NM in January and looking at Jan 2007 prices, a one way ticket was £600, but a round trip was only £480!