There are a sea of clothing manufacturers online. Many of them are listed in lists and directories. You will need to search terms like garment, apparel, clothing, fabric, etc. You might also want to post your needs in a forum online and see what companies others might recommend.
When looking for a manufacturer, first nail down all of the specifics that you need. Do you need silkscreening? Is this for t-shirts, childrens clothing, fleece, suits, etc? These types of specifics can help you narrow down a certain type of manufacturer. There are clothing companies that specialize online in t-shirts and silk screening and there are other that specialize in more complex clothing like coats and jackets.
Choosing a manufacturer in the United States will of course be more expensive than working with a foreign manufacturer. However, foreign manufacturers usually have high order minimums and will not work with someone with a small order.
Another way to find an apparel manufacturer is to post your job on a job board for manufacturers and let them bid on the job. Try to weed out manufacturers that have negative reviews or negative feedback. When you start, it is best to work with a manufacturer that has good reviews or feedback.
Make sure you evaluate several bids on your job before you decide who to work with. If you are in the US or Canada, its also wise to start working with a manufacturer in the US or Canada before you consider working with foreign manufacturers.