Keep an eye on the auctions in-game, and on the selling threads here. Every now & then one of the current ones pop up there.
MindArk also creates new ones occasionally, perhaps once a year or so. The new ones, when they are created, are also usually listed on the auction when they go up for sale.
If you are very serious about buying one, contact some of the current land owners; one or more of them may be willing to sell to you. If you haven't researched it yet, I would strongly suggest doing some reasearch on the prices for the last few that sold first before contacting anyone.