Bob
The posters were very popular a few years back but as all things of that nature they stopped being a big seller and were passed on down to smaller retailers. My guess is that if you want find a poster of that type that you try a head shop or try a local bookseller. I do know that when this item was popular there was a book published that had page after page of these illusions to peruse. There actually is a particular name for this type of illusion. I will see if I can find out what it is called and post it here. I think it might actually help if a person could get a hold of one of these posters and practice with it in order to develop the vision needed to see the blip page.
To get a blip page maybe try pretending that you are looking through the book at the floor below you. That works for me.
Play with PhotoFocus state. Another way to get a feel for the PhotoFocus state is an exercise from the martial arts - get a friend, stand several feet away from them, and as you gaze at them try to take in their whole body from head to toe without concentrating on any one feature. You're training your peripheral perceptions. When driving, occasionally see if you can see out the windshield and at the same time see out the rearview and side mirrors. Make a game of it, keep it light.