Most ADCs seem to be limited to +/-10V signals.
For instance, this Anaog Devices part
http://www.analog.com/static/imported-files/data_sheets/AD7656_7657_7658.pdf
uses +/-15V supply, Performs 6 simultaneus conversions, and looks to have good performance #s.
So take the +/-15 input, get some trimmed resistors to divide it down by 1/3, and off you go.
They have a good parametric search feature so you can find a part that meets your needs.