ERIC is a great resource for finding lesson plans. To search for these, you'll have to do so from the Advanced Search, as you cannot select the options you'll need to in either Basic Search or from the Results page.
Type the topic of your lesson plan into the search boxes on the top. If you're having trouble finding relevant results, you can try broadening your terms (ex: from "solar system" to "space" or "astronomy") or narrowing them.
Then, scroll down to the bottom of the page, and you'll find two options that will be helpful: Education Level, and Intended Audience. You can see them outlined in red below:
For Education Level, you'll want to choose your group of students whether they are first graders, high schoolers, or adult learners. There are a myriad of options here, so you'll probably want to select all that apply. To do so, simply hold 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.
For Intended Audience, there are two options you'll want to select if you are searching for lesson plan in particular: Practitioners and Teachers. As with the Education Level, 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.
Click "Search" at the bottom of the page!