I regularly monitor Nikon inventory in the US. It's clear that NikonUSA released a handful of 500mm lenses this past week, but they mostly got gobbled up as fast as they hit the shelves. A couple weeks previous to that NikonUSA released a handful of 200-400mm lenses, and a few of them remain in stock around the country, though I suspect they'll disappear soon, too. What it looks like to me is that Nikon has a monthly rotating system of exotic lens production. I wonder what we'll get next month, the 300mm or the 600mm?
But here's the key to getting out-of-stock lenses, I think: find a local dealer that carries Nikon Pro gear and that doesn't have any outstanding orders with NikonUSA for the product you want. Get them to place an order with NikonUSA for the lens. This order generally has to not just be one of the typical online orders, but has to be upgraded to the store's Nikon rep's attention. Why does this work? Legally, if Nikon gets 200 lenses they can't distribute them all to Dealer Z and none to Dealers A through Y. While a few high volume, high commitment dealers do get extra distribution, NikonUSA has always allocated relatively evenly amongst dealers of the same rank. The reps control the last bit of allocation. Those allocations first go to stores that the rep knows actually have an order in place. In the current economy, many dealers won't order the really expensive goods for stock unless they know they have an order in hand to ship to, thus, the hard-to-get lenses rarely show up on a dealers shelf waiting for a buyer.
So. Make friends with a good dealer. Establish a buying relationship with them. Make sure that they don't have other orders already in the queue (it's unlikely that your local dealer would get three 500mm's all at once to fulfill their outstanding orders when an item is still being allocated).
If you want to play the Amazon/Adorama/B&H Lottery to get hard-to-find lenses, try www.nowinstock.net and be really fast on the click-through. But a local dealer is a better bet right now until Nikon gets things back in balance (which, of course, currently appears to be "never" ;~).