There was a total of five IMAX 1 Sheets for SPECTRE between all the white, grey, Coming Soon and dated variants.
What makes it harder still to find these is that, at the time of most interest (the film's release), most ebay sellers decide to use a stock image, so there has to be much conversation back and forth to ensure they know what they are selling.
And, of course, one has to buy At The Time, as otherwise, they disappear...