One device that might work for you is the Behringer Ultramatch Pro
SRC2496. It's designed to be a sample rate and word length converter,
but it has both analog and digital inputs and outputs, so you can use
it as an A/D converter (as well as n D/A converter should you need
it). It's pretty cheap, about $150, and looks like it will give you
what you're looking for.
http://www.behringer.com/SRC2496/index.cfm?lang=ENG
I own a SCR2496 and I played with it spending 350$ to upgrade it....
Originally it sounded good, better then I thought, expecially the DA stage.
After the upgrade the results are incredible! But I can't get to cancel the
Behringer sign on the front... :-)
F.