When I began making jewelry and was still a beginner, I purchased as many parts as possible, including the findings. This way, when I made earrings I wouldn't have to worry about constructing the ear wires. As time went on, though, and I became more experienced, I made more and more parts for my pieces. These days, I make everything from scratch if I can (obviously, I can't craft my own Swarovski crystals).
Now that I know how to make my own earring hooks, they're actually fairly simple to design. The trick is to get them even so you don't have one earring that's longer than the other.
Given that I've made probably about a thousand of these hooks, I'm doing a little experiment and am seeing if I can sell a few supplies on Etsy. Until now, I've only sold finished products that are ready to wear, but many artists come to the site to look for inexpensive supplies -- like earring hooks.
This may not be my most exciting venture, but it's these little parts that are necessary for making jewelry. Where would a jewelry artist be without jumprings? Or a painter without brushes, for that matter. I'm always trying new things to improve my business, so we'll see how this goes!