Where do I find a list of Arduino DIY shields
Have a google for "Arduino Shield List" or similar, here are a couple of good links:

Site totally dedicated to Arduino shields and their pinouts:
shieldlist.org

Arduino compatible hardware:
List of Compatible Hardware for Arduino



To find out about the schematics and parts needed for the shield, it will depend mainly on the shield maker. If you follow the link on the shield page to the makers site, they will usually have this information in varying detail. 

I picked a couple of examples at random, the "Danger Shield" from Seeed Studio and the "Ethernet Shield" from Adafruit:

On the shieldlist page, there was a link to a Wiki with "make it" and "use it" headings detailing components and code:
Danger Shield Wiki
Also there was a link to the schematics and design files:
Danger Shield firmware and board design  

Adafuit had "make it" and "use it" pages along with these downloads:
Ethernet Shield schematic and library