It may depend on what the purpose of using a symbol for "yes" and "no" is.  I think written words are certainly a good way to go, but I don't like to give up the "real estate" on a communication board or device for yes/no, when there are often so many other ways a student can indicate yes and no (gesture, facial expression, sign, blink, etc.) that are more efficient and still effective.

I have found these resources very helpful when thinking about yes and no:

From Project Core:  https://www.dropbox.com/s/dl/h...

From Kate Ahern:  http://teachinglearnerswithmul...

Some addition supports for Tangible Symbols can be found at the Project-core website and at the Texas School for the Blind and Visually Impaired. If you do a search here in the forum under Tangible and/or Tactile symbols, you%u2019ll find all the posts that people have written.

Also here in the forum, look under the collections. There are a few articles pulled together under a Tangible Symbol collection. You%u2019ll also see references and links to the Project Core website and others

Hi!  I am actually really relieved to hear that someone is struggling with the same issue that we are. I work in an Early Childhood Special Ed preschool and have several students needing either PECS or more advanced dynamic systems (eye gaze, Proloquo) for augmentative/alternative communication needs.  For "no", we've tried the big red X, a stop sign (with the word "stop" removed and "no" in its place), and the red circle with a diagonal line through the middle... the reason being all these abstract symbols are used in the real world, on various signs, packaging, etc. to represent some kind of protest, whether it be something along the lines of "don't touch this", "don't go here", etc.  For "yes", we had tried the green checkmark with a smiley face symbol.  The green/red colors  are at least consistently used throughout our program and our classrooms, for stop/go, yes/no, ready/not ready, time for/all done, and to represent the general idea of positive/negative.  And green is used a bit in the "real world" to indicate types of "yes".  But nothing has really stuck as a consistent, concrete, yes/no.  

I've started to at least try to be consistent with the symbols on the communication systems that the kids will be using, or are using.  Below is what the default yes/no symbols look like on Proloquo2Go.  

So I guess we are in the same boat as you!  I hope you find something that suits your needs and just know there are more of us out there searching for answers too. :) Thanks for posting, it really caught my eye.