Many websites are specifically devoted (like this one) to listing free product offers. On these websites you'll find a number of links to free product samples. Most of these websites are user driven so you can share your own free product finds or find out what other people enjoyed about the free sample offers they are recommending. You can easily find these websites by doing a search for "free samples."
A second option is to check the websites of all your favorite retailers that sell nationally where you'll often be able to find free product promotions. The most common example of this is Wal-Mart, which regularly offers free samples for many brand-name products.
You can also go directly to the source of the product. Manufacturers testing new products will often offer free samples to get feedback and test demand. You can find these under the homepages of a company's website. Look for links to special free offers or promotions. So visit your favorite companies' websites periodically to see what free deals they offer. Here is a list of pictures of free samples: http://yofreesamples.com/
Last, try a simple internet search. If you are looking for a specific product or sample but don't know which manufacturer or distributor to look under, then you can simply type in the name of the product you are looking for along with the term "free sample" into your search engine.