UPDATE FEB 2019: After various districts were dissolved, it was announced they they will be put up for auction once Decentraland launches in Q2 2019. You can read more about this and locate the unowned LANDs here.
UPDATE DEC 2018: The 2nd Decentraland auction will happen in the 2nd week of December 2018. More info here
At the time of writing (July 2018) there has been no announcement from DCL with regards to buying unowned LAND in Decentraland. It is still unclear whether there will ever be another auction, or the unsold LAND parcels will be made unavailable to purchase.