If it's something I've made before, I usually use my experience more than written plans. If its something that I think I can build from looking at a picture, then I create my own "plans". On larger items, I tend to search for free plans on the internet. I've also looked other projects posted by woodworkers and then ask if they'd be willing to share any advice or info. I have purchased plans from Rockler & Woodcraft, though their plans can be found on many other sites as well.
"Measuring is the enemy of accuracy." Chris Schwartz