It seems like recently we’ve gotten quite a few questions regarding finding patterns for making things. Patterns are very helpful for the beginning sewer, because not only do they provide a ready-made pattern for you to make, often with multiple sizes, but they give you step-by-step instructions for making the item. M-sewing and Burda have a large selection of free patterns (Burda also has a lot of patterns for sale, and they tend to be a little more stylish than other brands). McCalls is the main clothing pattern company. They have several lines, including Kwik-Sew (easy patterns for beginners) and Vogue (classier patterns with a lot of coats and the like). You can buy patterns online or in fabric stores. You can often find vintage, out of print patterns on ebay or Amazon.
However, don’t feel restrained by the pattern! Patterns are made for people who are perfect proportions and perfectly average height, and nobody really fits that mold. Do a test run for any pattern to know what you need to alter for a better fit.
I personally make almost all my own patterns, although I started out sewing exclusively from patterns. This is because 1) patterns are way expensive and 2) most cosplay items require altering a pattern anyway, and a lot of cosplay items I’ve made would be virtually impossible to find a pattern for. If you want to learn to draft your own patterns, this is an awesome youtube series that introduces you to patternmaking. Buying or making a dressform will also allow you to drape some items, which is immensely helpful for patternmaking. Finally, you can buy cheap clothing and tear it apart to “steal” the pattern - if you need just a set-in sleeve or a collar, this is much cheaper than buying a whole pattern!