In the left hand column on this page, scroll down until you see "Trade Skills - By Trade" and click on leatherworking when you get to the page. This will give you a list of all the available leatherworking patterns. The ones like "rugged leather pants" which have a pattern listed underneath them in colored letters are drops - you can sometimes find these at the auction house, or you can click on the pattern on the same page to find out where it drops and go farm it. The ones like Deviate Scale Cloak which have a pattern listed under them in white letters can be bought from vendors or obtained as quest rewards - click on the pattern to see which vendor you can buy it from.
Alternatively, type in "patterns" on the search box in the left hand column. This will give both tailoring and leatherworking patterns, but it also shows who sells them, where they are sold, whether they are quest rewards, and whether they are world or static drops.