My first reaction is "good lucK". Your designs are protected by copyright, however what counts as infringement is subjective, like how much of another work can you publish without permission or it infriniging.
You can find out who owns their current netblock via going to www.netcraft.com and typing in their domain in the "What is that site running?" box in the upper left hand region of the site.
You can also query the Whois database and find out who registered the domain and the various contact information and attempt to contact with a cease and desist. Also, often the technical contact information is their ISP info and whom to contact about infringement.
You'll need to have a lawyer draft up a cease and desist letter on their letter head and send to both the infringer, and the infringer's ISP. The ISP will usually cave as having other's copyrighted works is typically a violoation of TOS.
However, if they rebuff your claim and decide to fight, its going to proably cost several thousand dollars to file suit, so unless its worth $5 - 10k to you, proably not worth your time to persue.