Well, I pulled that number from a couple of articles. Here is one:
http://www.wrmea.com/html/us_aid_to_israel.htm
Admittedly, many of the places I found this number (30%) are Palestinian or pro-peace or whatever, but the numbers are there.
The directory for the 2004 FY US Internation Affairs request is here:
http://fas.org/asmp/profiles/aid/fy2004/
I can dig up some other stuff too if you want.