Though I've said it in previous posts, it bears repeating that this alone cannot be the only way to explore sailor's clothing.
One of the best sources for eighteenth century sailors' slop clothes comes from the wreck of the General Carlton of Whitby, which sank in 1785 off the European coast while carrying a load of pine tar. The casks of tar burst when she hit the bottom, and coated much of the ship. When she was discovered in the 1980s, a wealth of original artifacts was recovered.
You can read more about the wreck here.
Another excellent source is written documents. For this example, I point to the blog With Messrs. Wallace, Davidson and Johnson. The blog follows primary source documents from a merchant firm in colonial Baltimore. Of particular interest in the long list of goods sold as "slops" to crews in Baltimore!
When we combine material, textual, and visual primary sources, it is much easier to round out our image of common clothing in the eighteenth century.