We got ours off of eBay. Outside of the monastery, any Westvleteren is sold is sold on the grey market (another reason for the high prices). All the trappist monasteries brewing beer are non-profit organizations. Most of them take any extra profits and use them for charitable actions. But the Westvleteren monastery only produces enough beer to keep the brewery open, so unlike the others they don't produce year round, and they only produce a very small amount of beer.
In addition, the only place to buy the beer is at the monastery (or at the pub/shop across the street from the monastery). If you buy a whole case, the put your name on a list and won't let you buy any more until the next year. They try (not hard) to keep people from reselling it for personal gain. So I guess if you got caught they wouldn't sell you any more beer. But that's not really feasible to do in this age.
You can find it on eBay listed under collectible bottles. It's not entirely legal to sell alcohol on eBay, but there is a loophole. You can't sell the beer, but you can sell the bottle for its collector value. You just need to stipulate in the auction that you are selling a still capped bottle which just happens to contain alcohol in it. But remember, you're buying the bottle, not the beer
Just find a reputable seller that's sold it in the past and check their reviews. That way you'll see if they are legit and will really deliver the beer (and that they haven't screwed it up in the past). Keep in mind that it will probably take a while to get to you since most of the sellers live in Belgium. International mail takes awhile, and it also has to clear through customs. Ours took a few weeks, maybe a month at the most.