How do I find a doctor who will prescribe bexarotene, sold under the name targretin, for off label use against alzheimer's disease?

No human data. The data is only in mice and human trials are yet to start, targetrin can have serious side effects in patients. Treating diseases in mice does not often translate to success in humas.....Wait for more data.
Find a Good Doc. For my family i would concern myself with finding a good physician that is accomplished in dealing with dementia. It is likely that s/he will be aware of different clinical trials currently underway & possibly point the way to enrolling in a study. That would be one safe way to avail yourself of treatments not yet in the mainstream.
Wait for more data! The result has not yet been independently confirmed. When results look "too good to be true", they usually arent true.