Lee seems to always be behind demand. I recently got one but I had to wait a few months in back order limbo. I got mine from B&H. While you are at it you might as well order a filter holder that can hold the 4x4 size of this filter. Lee makes the Lee Foundation Kit and Cokin makes the Z Pro. Both are compatible with 4-inch wide rectangular/square filters. The Lee holder also suffers from constant back order status. If you want the Lee get on the list so when they do go into stock you will get one.
Be aware that the Big Stopper does create a rather strong blue color cast and can hot spot in the center. Both can be addressed in post processing though.
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I put an order in with 2filter.com. They seem like a tiny, off the map outfit but I've purchased from them before and have had a great experience. I figure they'll get Lee filters back in stock as fast as anyone else. B&H wouldn't even allow me to put an order in.
There are some available on eBay and if I get desperate it's always an option, if I'm willing to pay the significant upcharge.
Claudia, I ordered a Big Stopper, the filter holder & wide angle adapter last night (1/20/13) from Unique Photo and just got an email from them stating that my order has shipped via Priority Mail. I thought the price was really good too @ $145.00. If you're in a hurry to get one you might want to check and see if they still have them in stock.
Thanks to everyone for the leads and advice. As it happens, I was at my local shop today (Pictureline in Salt Lake City) and asked how long a wait for a Big Stopper. Today must be my lucky day because they had 3 or 4 in stock. I bought it.