Public Access (Free) Patent Websites:
The easiest is to search is Google Patents, which has the most efficient search engine. Next is Patent Lens, where you can view foreign, US patents and patent applications. The U.S. Patent and Trademark Office search engine is less user-friendly. However, once you have determined the likely classification number(s) for your patent, The U.S. Patent and Trademark Office site is the most authoritative for searching by Patent Class.
Other Patent Sites - Not Free
The Cooper Union Library
7 E 7th St.
New York, NY