Freebie sites Oh Yes It's Free and Pretty Thrifty list free lip gloss samples on their websites. Makeup companies such as Sephora and women's magazine Allure offer free beauty samples that oftentimes include lip gloss. Other places to search for samples are the Target Sample Spot or About Free Samples.
Pretty Thrifty has a page devoted to finding free beauty samples for the lips. Their most recent lip gloss sample offer is the bareMinerals Marvelous Moxie Lipgloss, and they also list the Nars Cosmetics Lip Pencil from Sephora.
Oh Yes It's Free, Target Sample Spot and About Free Samples require more searching, and lip gloss offers are not always listed, but it's worth checking the sites every so often as the offerings fluctuate and may change to include lip gloss. Sephora and Allure work in a similar way.