The best way to search for trademarks is through the United States Patent and Trademark Office online or at one of its physical Patent and Trademark Resource Centers. These centers are located at libraries around the United States, and the USPTO online can be searched to determine which ones.
The USPTO provides information on the process of acquiring patents and trademarks and offers a database that includes all patents and trademarks filed since 1790. Patents and trademarks are searchable by many fields, but those filed before 1975 are only available by issue date, patent number and current us classification.
The physical PTRCs have employees there to help in understanding the process of applying for and searching patents and trademarks and can put you in touch with the appropriate attorneys as well.