Two sites that are especially useful during this current ebook turmoil:

Inkmesh ebook search engine
Very quick search. Cannot limit results to specific formats or stores.
AddALL Ebook price comparison
Slower search but you can specify formats (e.g. ePub, PDF). Can exclude sources (e.g. Barnes & Noble since their ePub DRM won't work on the eDGe). It notes whether or not the offering has DRM.

I tend to use them both in case one comes up with a nice result the other missed.