We have used Knowles miniature microphones <http://www.knowles.com/search/product.htm?x_sub_cat_id=8> for many years for many purposes, including in-ear sound pressure recordings. We used a probe tube (as do most other people who make such recordings), which can cause noise floor problems at high frequencies. The fairly new FG series of microphones from Knowles is small enough that you could probably put the entire microphone in the ear canal without affecting the sound field too much. (I see that the GA series is even smaller, but we have not used them yet.) I assume that this is for human subjects.
Mike Ravicz, Eaton-Peabody Lab., Mass. Eye & Ear Infirmary, Boston MA 02114
+1 (617) 573-5591; +1 (617) 720-4408 FAX
"In theory, there is no difference between theory and fact; but in fact, there is." - someone