Like Xaelia, I know of no independent source to find out about percentages of accepted applicants from each undergrad.
As you conduct your own research, just be aware that the numbers reported by the individual undergrad schools are not necc. an accurate portrayal. Ex: a school that reports a lower acceptance rate may counsel and prepare LOR committee packets for *every* student who expresses an interest in medicine, regardless of test scores/GPA, while other undergrads may be more selective in who they support for med school.
Also, I think the numbers for 2003 accepted applicants on mdapplicants.com are a little on the high side right now. Once May 15th rolls around and the wait lists start moving, I think some of the MCAT scores and GPAs of the accepted applicants will be more moderate.