The iTunes Store Web Service Search API (pdf), although old and incomplete, documents how to search for content in the iTunes store and returns a JSON string with results.
The iTunes Store Web Service Search API allows you to place search fields in your website to search for
content within the iTunes Store. You can search for a variety of iTunes Store content; including movies,
podcasts, music, music videos, audiobooks, and TV shows. You can also call an ID-based lookup request to
create mappings between your content library and the iTunes Store content library.
Queries are sent to base url:
http://ax.phobos.apple.com.edgesuite.net/WebObjects/MZStoreServices.woa/wa/wsSearch?parameterkeyvalue
Where parameterkeyvalue can be one or more parameter key and value pairs indicating the details of your
query.