The best place to buy a cappella arrangements

I have created a robotic singing group that performs every Halloween.

It’s too difficult to describe, but you can see it here:

http://www.youtube.com/watch?v=hAt2meAGrwg

I am thinking of doing several new parodies for 2010. Some of the songs I want to do are:

Stormy Weather
Dream a little Dream of Me
Dancing Cheek To Cheek
What a Day for a Daydream

I am also looking for sheet music for "Shadows, Ghosts and Bones" by Stormy Weather.

I want to find a capella versions of these songs so I can use the harmonies and record my own lyrics. I want interesting parts like things that would be arranged for a group like Rockapella where each part is unique and doing a very different thing than the others, but they all mix together to sound great!

Where, in your opinion is the best place to purchase the most interesting a cappella arrangements of any song, but especially those that I have listed?

It would be great if wherever I buy from would let me preview some of the score or listen to some of the arrangement even in MIDI form.

Thank you for your help!