ERIC is a great resource for finding lesson plans. On the Advanced Search interface, scroll down to the bottom of the page and locate Education Level and Intended Audience boxes. For Education Level, you'll want to choose your group of students. For Intended Audience, there are two options you'll want to select if you are searching for lesson plan in particular: Practitioners and Teachers. You can select both of these options by holding down the Ctrl key (on Macs, you'll want to click the Command key) as you are clicking on the options you'd like to select. They will all appear in blue once you've selected them.