Stl114 - The alternate stereo mix of WSOP was a limited edition single, one side cut at 33.3 and the other at 45 rpm. The record itself has a Regal Zonophone label on it. Tim DeParavicini is the head of EAR electronics. He is noted for his modified studio tape recorders and tube microphones and such.
I purchased the single from Acoustic Sounds a few years ago, maybe 2007. I don't remember the price, but I don't think it was too much. As far as I can see, it's gone. I can't imagine finding a used copy, as I don't see why anyone who liked the song enough to purchase it would now part with it.
BTW - I don't know if this is true, but I read somewhere that WSOP is STILL the most played song in British pubs. Pretty incredible for a 1967 song, although I still think the best rock music ever produced was from about 67-69. Joplin, Hendrix, Creedence, The Band, Sly, Zeppelin 1... The list goes on.